Hi Nick,
There is a company in Newport called Osaka Newport who service SC430's, I have not used them yet, but have spoken to them on the phone, they are trained Lexus and Toyoto people. Phone 01633 223670.
You can always try Lexus Bristol, Antoine is the Service Manager, worth calling him for a price first, I have found him most knowledgeable and helpful and price wise not to far away from non Lexus garages.
My SC430 is due an MOT in two weeks time and has had four 10,000 mile services in 20 years - and still has only 10,000 miles on the clock - so I am thinking that an oil and filter change should do for this year.
I also live in Gloucestershire, please could you inform as to where your car is storaged as my 2003 SC430 is also in storage locally and I would like to find somewhere quite not so expensive. Cheers Peter