Hi Mick T,
I agree with the other replies. I had mine done @80K miles and although the gear changes were OK before but after the oil/filter change, the changes were imperceptible and pickup much improved. Bristol transmissions sell a kit with the correct oil and the replacement filter unit and fasteners.
(I have no connection with Bristol transmissions other than a satisfied customer).
I would suggest you have the work done at a competent independent as it is quite a tricky operation to fully drain and re-fill. The correct temperatures must be achieved whilst re-filling. I also had the transmission software upgraded at the same time ("flashed".)
Regards, Irish Pappy