I have a potterton kingfisher mf heating system and want to change it as it's very exspensive to run, I've been told to locate the zone valve but just can not find it, been in the loft, airing cupboard and all around the boiler, might it be under the floor? And will the definitely be one?

How many thermostats do you have? They regulate the zone valve through the control board. With only 1 t'stat, doubt you have a zone valve.

Google "potterton kingfisher mf heating system". You may find additional info.
We have no idea if you have one or where it might be. That is not a US system so it further complicates issues.