Thanks, found it under the floorboards in the bedroom above the boiler.
Having confirmed this isn't working (by turning heating/hot water settings to hot water only and finding both pipes leaving the valve get very hot), I find it's not the only problem. The hot water is getting to the point...
Silly question but where is the diverter valve?
There are only two pipes attached to the boiler (Halstead 50) which suggests one out and one return, in which case the diverter must be after the pump and under the bedroom floorboards somewhere?? :?: