I want to get back on super unleaded but I still have under half a tank of regular left. I'm taking the other half to gatwick from mansfield tomorrow but I don't want to fill up with regular then spend the next couple of weeks running it empty. Will it make any difference to the engine if I top up with super ?

You'll just have a graduated rise to SUL's improved octane value.  The engine adapts combustion by combustion, so just fill up and go.  Be aware it will take a couple of tanks to feel the real benefit though (long term adaption).

Agreed; if you mix different octane (or citane for diesels) rating fuels you get a rating that's proportionate to the mixture.

I want to get back on super unleaded but I still have under half a tank of regular left. I'm taking the other half to gatwick from mansfield tomorrow but I don't want to fill up with regular then spend the next couple of weeks running it empty. Will it make any difference to the engine if I top up with super ?

No difference to the engine, only your wallet.

I'd stick with the regular too, not sure what the OP is really trying to gain, especially in real world driving.