Just to finish - registered with a couple of online breakers, got a couple of offers, Terry from Neo Bros phoned yesterday morning, I called him back at 4.20 pm, glass arrived at 10 this morning via FedEx. £21.60 inc del. and VAT. Excellent service.
As per sgould's and Audax's pointers above, new mirror on in 15 minutes. Lubrication is essential to be sure it clips home fully, I used GT85 bike spray, nearest thing to hand. Getting the old one out is easy if there's no glass there, the backing plate flexes to access the springs even in this cold weather.
Thanks again chaps.