To convert to export style higher 'bars I assume the control cables will need replacing with longer items? If so is there a good source of ready to fit ones available?
Welcome, Old-Nail
Originally there would have been a set of 'export' cables to match the export bars (with their own specific part numbers), and if you mention export type to a BSA parts supplier then they should know which length cables you will need (provided they are standard type A65 export bars?)
However if you want to, you can obtain cables from specialist cable suppliers like Venhills:
http://www.venhill.co.uk/ as they will have records of all the various cable types and sizes, and you can verify how much extra length they need to be by measuring.
Will the wiring to the horn etc. reach the higher bars or does that part of the harness need extending also?
The original switch wiring may be long enough, but it's difficult to say on an old bike without seeing it, look inside the headlamp shell and see if there is any spare length switch harness inside that could be pulled out? Or you could re-wire the switches?
And finally, where would I get a pair of those under tank reflectors as fitted to export models as standard?
I think the type you need is the Lucas 57173 (BSA part 19-1035)?
I suggest you try C&D Autos for those and any other spares you may need (cables?)
http://pages.eidosnet.co.uk/~canddautos/Ooh!... and finally finally, if I decided to just revert to the UK style petrol tank of the correct 1970 A65L type, which other models was that tank used on, or was the tank specific to that one machine?
It would also have been used on the very similar UK A65T (Thunderbolt) model, and maybe the A50 model as well?
There were three tanks available for 1970 I think, the steel US 2 Gal. tank, UK and general export 4 Gal. tank and also a fibreglass tank ('70 US A65F?)?