Yes Tudor - IIRC Bob Prichard wrote about this in an issue of COL some months ago (or was it elsewhere - but it was surely Bob).
It should be well established in the data and where better to look but COL.
I have a pet theory that the thunder totals have held up best in Lincolnshire. Why? Just off the top of my head but if we have more SW flows in summer half year (i.e. not W'ly) then there's the issue of convergence with the sea breeze on fine afternoons. SE England too near to high pressure areas to the south-east. I think we all know we don't get the southerly Spanish plume events - not in the SE anyway.
There's a journal paper that tries to look at thunder frequencies in the official network.
....they found a decline over time. However, it was pointed out by Stephen Burt (who became one of the authors) that this was partly an artefact of the reduced staffing of such stations, hence my point about the value of consistent COL sites with vigilant observers. BTW, the Geoscience Data Journal, publ by RMetS is well worth browsing (via link above), not least because all papers are open access.