Thanks for the pointers so far, it does seem like the consensus (both here and in previous threads across multiple sites) is to take it slowly with regular partial changes. Generally speaking, maintenance has been carried out on it (I have FSH with it), but just the ATF especially has been neglected, because 'lifetime' fluids, no doubt.
Othren, I do intend to change the filter and clean the pan etc. unfortunately, the lead times are all kinda long, so that'll probs have to wait until September. If anyone has any advice on the dipstick tube removal, that would be great (or should that go in a new thread?). Also, thanks for the link to the pump, looks pretty snazzy
I'll make a start in a bit and get a couple of litres out today (hopefully).