Might have sorted it.
I took it off during the week and peeled off the old tape. Then popped it, along with the roll of tape, into the central heating boiler cupboard for an hour to warm it up nicely. Then taped it up, popped it back into the cupboard for another hour, took it out and left it for a few days.
Got up this morning, and wiped the bumper section down and warmed the diffuser up for an hour again, then peeled off the backing and heated again, then fitted it. Really seems to have bonded well, but it did that last time and only started to come away after a few days, so fingers crossed!
The 3M website that it gains 50% of it's bond within 20 minutes, so hopefully should be alright. Looks great with the exhaust tips