ASAP supplies give great service, nice people and really helpful. They are experts on the Thorneycroft engines and a whole lot of other stuff.
But, if you need an oil and fuel filters find a good motor factors and buy six of each in one trip. They should cost you about one quarter the price in a chandlers and it is always good to have a spare or two.
Thornycroft / AMC in Preston seem to hold everything for the 1.5 and 1.8 litre engines. ASAP are good too. And a lot of the run of the mill stuff you can get from motor factors and some Rover and LDV dealers. Lucas CAV service depots all over the UK do the fuel injection bits too, as well as exchange starters and alternators. Heat exchangers are usually Bowman and the sea water pump is a common Jabsco (agents everywhere). Parts availability is excellent all round, and at lower prices than for other marine diesels.