Bright yellow ladybird about 1cm long
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of Majerus & Kearns' "Ladybirds" (Naturalists' Handbook series, so not extensive, and pre-Harlequin), there are illustrations of 2-Spot Ladybirds at various stages after emergence from the pupa. Your photos match the pictures of individuals up to 6 hours old.
I don't know how many other species show this sort of colour development, though.
At 1cm long this is far too large to be a Two-spot ladybird. However it does look like a ladybird of some type so I wonder if it's a plain-coloured variant of the Harlequin which reaches about 8mm long.
That the size was possibly too large, but "about 1cm" (no offence intended to the poster) is not necessarily accurate, without a scale on the picture.
I've looked for Harlequin variants of this colour, but all seem to have either spots or cryptic patterns: none appear unmarked.
Nonetheless, I bow to your experience, and happily withdraw my suggested identification, though I don't think I can easily delete it.
The legs look black which would discount the harlequin and the estimated size of 1cm is close enough to the 8mm size of a larger 7-Spot (Coccinella septempunctata) also the pronotum pattern fits. Definitely Teneral, keep an eye on it and within a day you should start seeing the spot patternation develop.
Let us know!
Thanks for all your information. My measurement is probably not very acurate, and I always forget to put a scale in my photos, I was in a hurry to photograph it before it flew away. Unfortunately I didn't keep it to beable to say if it has changed colour, but i'll keep a look out for any more.
Lat/Lng: 52.073241650972, -2.8055799007416
OS grid ref: SO448419