Thanks again, leg off this morning - first time for me and pretty easy once you know how.
The impeller actually looks in perfect condition - clueless as to how long it's been in there but obviously I will change it anyway with a new gasket. Rang Black dog (Dan) very helpful as everyone says.
'Stat is out, really clean and tested - working fine.
Couple of points I've learned from doing it - the detent spring on the yoke does not need removing as per manual, the coupler pops off fine (this helps as the two bolts securing the spring are a PITA to access).
And a strip of gaffer tape left wrapped around the shift shaft allows you to measure down to the reverse lock actuator so it can be replaced in exactly the same position.