Most main dealers don't know how, or simply don't want, to care for an autobox of this age so, before you shell out on the fluid, and as you are in the UK, consider having the EDT cleaning and transfer method carried out.
See links to the EDT site (post #13) in Tenuta's thread:
Wow - changed auto gearbox oil and now like new ...and check their locations as you may have an independent close to you.