radiators on when heating turned off

My upstairs radiators are getting hot when the heating is switched off in the airing cupboard.
Yesterday I fitted a new motorised valve, pump and removed the radiators, flushed them out and refitted them.
I refilled the system with inhibitor, flashed it up the heating still came on.
I a suspicion that it might be the anti-gravity valve but not sure where to locate it, any advice welcome.

Any history here? Has it always done it?
I assume you have a fully pumped system (3-port valve and all that), so piping error is suspected. There was another thread here today on the same subject.

It only seems to have started with this problem since I turned the heating off for this summer.
Originally, I thought that the 3-port valve was leaking or stuck letting hot water pass into the heating system when switched to hot water only and with the house being 22 years old I thought I might aswell replace it anyway.
The piping is the same as it has always been, only being altered to fit a new tank but that was just to line it up.

Probablt reverse circulation due to incorrect positioning of return pipes.To check allow htg to completely cool & switch on dhw only,go to an upstairs rad & check which way water is flowing in to it.Then switch on htg if flow changes then you have reverse circulation if it stays same suspect 3 port valve