Definitely from either Thornycroft or http://www.asapsupplies.com/ who are usually cheaper than T/Croft but obviously cannot supply specific engine spares. For example the ASAP Heater plugs are half the price of T/Croft.....
Like others before me, I can thoroughly recommend ASAP.
My engine is a Thorneycroft 154, and it decided to be difficult during the annual summer cruise. We were in Campbeltown, not exactly convenient for post/carrier, etc or even diagnosing the parts required. They faxed me exploded diagrams of the relevant units, identified the precise part over the following phone call and then sent off the part needed. It was waiting for us at our next port of call.
Excellent service, knowledgeable answers on phone and quick,accurate delivery.