Having done a fair bit of reading over the last few days and having received many helpful comments from other birders, I now feel a bit better qualified to comment on the identification of this shrike. In short, I think it is a Brown Shrike, intend to submit a description to the BBRC, but suspect that we may not quite have enough to get it through. The only sure fire way to clinch these buggers is on wing-formula (see right - click on image to see it properly ). Actually, the Lizard bird represents a bit of a test case as to whether these birds can be accepted without in-hand measurements or clear photographs of the primaries (devine revenge for my post about field notes?). Typically Brown has 4-5 exposed primaries, Izzy 6-7 and Red-backed 7-8. Note - all these figures include P2, which isn't always visable. Overall, this generally leads to an impression of shorter primary projection in Brown Shrikes (a feature of our bird), but note how this is partly due to the wing formula. Also compare here and here to see why I think estimating primary projection in the field can be subjective (both photos are of the same Red-backed Shrike).
First-off, a bit of an overview of taxonomy of this and the confusion taxa. Basically there are four or maybe three races of Brown Shrike (the nominate (cristatus), lucionensis, superciliosus and confuses. The last one isn’t diagnosable and may represent an integrade between cristatus and lucionensis. Only cristatis is likely to get here (the others are mega far-east) and has breeding and wintering grounds overlapping with e.g. Radde's Warbler. Many of the typical Brown Shrike features (e.g. long tail) are accentuated in the eastern races, so those of you familar with Brown Shrikes from the far-east may not appreciate the extent to which they can be quite similar to other shrike species. There are four races of Isabelline Shrike. It all becomes bit of a 'mare naming them, as the old type specimen was attributed to a different race, so that race became the nominate (see Pearson 2000 Bull. B.O.C - also here). However, it’s also been argued that Pearson’s changes are not valid as the type was actually a hybrid (Panov 2009, Sandgrouse), so it might change again. I’ll stick to Pearson, in which case we have isabellinus (Daurian Shrike), phoenicuroides (Turkestan Shrike) and aranarius and tsadamensis. Only isabellinus and phoenicuroides are serious vagrancy cabdidates as the others are short-distance migrants from the far-east. Red-backed shrikes comprise three taxa: the nominate collurio (Europe), pallidifrons (Siberia) and kobylini (Caucasus and Crimea). The fourth: juxtus was the British type that’s now extinct. Anyway there's some good photos of eastern Shrikes here even though most represent races unlikely to get here.
Second-off – why is almost certainly not a red-backed shrike? Firstly, very few 1st winters or females have very dark, almost black ear coverts (see here for a typical one). However, red-backed shrikes, especially the eastern ones are a very variable and some females can look very like males. Males have black ear-coverts, and male-type females do too. It's conceivable that the odd female or juvenile does get quite dark, like the Scillies bird last year, which had a typical Red-backed Shrike wing-formula (see here - you'll need to login to surfbirds though - browner than ours though). More importantly - 1st year shrikes of both Brown and Red-backed have distinct pale fringes to the tertials (and greater coverts) and adults (second years?) only rarely do. This feature was noted in the field by several observers, suggesting our bird was a 1st winter. Other features in support of it being a first-year was the barring on the upper-tail coverts observed in the field. Adults also generally have darker-based bills and dark lores. 1st winter Red-backed Shrikes always have distinct heavy streaking on the back, lacking on Brown. The photos show only feint streaking and from field observations I'm confident that heavy streaking was lacking. Another feature in favour of Brown, was the observed clear-cut and bolder fringing on the coverts (see here - again login to surfbirds). The bird on top is a Brown, the lower Red-backed). Other supporting features in favour of Brown are the general tone of the bird and the lack of obvious grey nape.
This leaves isabellinus (Daurian Shrike) or phoenicuroides(Turkestan Shrike). I think the general tones of the bird rule out Daurian Shrike. They never really look anything like our bird. I think the serious sticking point is whether the bird was an usually brown Turkestan Shrike lacking the typical rusty tail, the latter being the very reliable feature, alhtough often this can be a bit subtle - see here for an example). In the field, I saw no evidence of an obvious rusty tail. The photos suggest the uppertail coverts and rump are slightly rusty and do show some contrast with the mantle, but the lower tail appears much browner. This is a pattern quite normal in brown shrikes, and not evident in photos of Isabelline Shrikes I've seen. Whether it's entirely unprecedented, I'm not sure of though. Two other features strongly suggestive of Brown over Turkestan are the very dark-brown ear coverts and very obvious pale supercilium. Again, I'm yet to see photos of a bird of either race with such a distinct supercilium, but welcome comments to the contrary.
Anybody who fancies a bit of further reading - I thoroughly recommend Tim Worfolk's article in Dutch Birding (2000, 22:323-362).