It took a while but a local firm came back with a quote of £10.80 each made up of £4.80 for a "Fruit of the Loom" T shirt plus £6 each for "heat transfer full colour large size logo".
I don't know whether that is a good quote or not.
Trish asked if we could have one made up as a sample but they will only do a minimum run of 6 so we have parked the idea at the moment.