This is how I would envisage it being done.... Which is based on Dunsleys layout for the neutralizer.

Natural gravity circulation operates until the top DHW section is heated and then a stat on the lower section energises the CH circulator.
A nc mv is fitted at X and a lo stat on stove return de-energises the mv at 45c preventing an induced cooling of the boiler.
Gravity circulation then resumes. ( Hopefully)