Sime Friendly Format 80e, replaced diverter valve but rads. still get hot?

Boiler: Sime Friendly Format 80e
Problem: Radiators get hot when DHW is run.

I had assumed this was the diverter valve, so ordered a new assy. (part: 6102806) and fitted. But it hasn't really fixed the problem, though it does seem to be marginally better.

When someone has a shower (say, a 10 minute duration) the radiators still get hot. It's as though the diverter valve is only allowing the shower to use some of the heat, and the rest is going round the CH circuit.

Could it be that the new diverter valve I fitted was faulty? Or maybe there is some other part that is causing the fault?

Thanks, Mike