Well the knack came from somewhere Kimpril.
I was on the point of going out to buy a 'Uniblade' fitting (adjustable) when I thought I will give it one more try. The seal had been sitting on the bathroom radiator covered in a towel to try to warm it enough to soften it and there was still a smear of washing up liquid where required on the shower door - sort of semi-dried.
Well, blow me, if the d*amned thing didn't go on with little resistance. Job done.