Have trawled a few bird club sites to expand the list:-
Cornwall - 433 (2011, as per David parker's comments)
Shetland (incl Fair Isle) - 431 (2008)
Devon - 423 (2009, with 2 pending)
Dorset - 417 (2010)
Kent - 415 (with 1 pending)
Essex - 390
Sussex - 384 (2002, with 4+ pending)
County Durham - 382 (with 3 pending)
Hampshire - 364 (2009, with 5 pending)
Lancashire - 354
Somerset - 352 (2008)
Cheshire - 351 (2009)
Surrey - 333 (2006)
Derbyshire - 319 (2011)
Wiltshire - 318 (2009)
Leicestershire & Rutland - 316 (2011)
Staffordshire - 313 (2008, with 1 pending)
Hertfordshire - 307 (2010, with 1 pending)
Warwickshire - 307 (2008)
Worcestershire - 300 (2008, with 3 pending)
West Midlands - 271 (2008)