James,
go to WOMA WEST OXFORDSHIRE MOTOR AUCTIONS, they have a website, i have bought loads of ex plod motors(used to work there)
and never ad a problem with any of them unless it was stated on the auction description, Tunnie has been there, they have police sales twice a month and the prices are good, however be warned you will be biddin against dealers, the advantge you have is you can go higher than them cos thy want to take at least a grand out of the car on resale, so thye wont bid higher that the profit margin allows,
if you have never been before would recommend you go a couple of times to see how it works before bidding!!
fittrf lpg to 2 ex plod senators and both went n to cover over 200,000 miles wothout anything major done to them
Omegatoy
P.S. the most sought after cars are the thames valley as they are always in the best condition and have service history with them, however they do command a premium ove rthe other forces as the dealers know!!