Thanks for the input & ideas everyone.
I check tyre pressure almost weekly- no issues there.
Previous owner of the wheels PM'd me & let me know he never hit a kurb or big pot hole etc and he experienced the same slight left hand pull (& there is not a mark on the wheels to suggest anything to the contrary) so that should eliminate buckled wheel.

I'll take it into wheels world and hopefully as kym suggested it will be castor...
If not, I don't suppose anyone has some lca's I could try before I shell out for some that I may not need?