Why can we only get heating when hot water on also now(Dual programer)?

We recently needed to replace head valve due to pump over run problem.(Boiler is Potterton Profile) On putting Potterton EP6000 programmer back onto backplate, screen was blank,so replaced this with new EP6002 onto existing backplate.
All worked fine for an hour after engineer left with CH & HW set to once. Then when I put both back to timed, found we could only get heating with hot water on at same time. Engineer thought it must be faulty head, so he came and replaced head,but no change. He then got his electrician pal to come and check all wiring to head just in case and they found all was OK. They checked and rechecked everything, moving room stat up and down and sliding cotrols on programmer seperately etc and all then worked fine for 8 days. Came home one day and pressed advance on CH, but nothing happened, tried turning up room stat but again,no joy.advanced HW, boiler fired up. So replaced the new programmer with another, but again no difference. Then replaced old Tower room stat with new Honeywell T6360 in case it was this. This controls temperature fine, but still only when CH & HW on together. As soon as you take off the HW and leave CH only, boiler shuts down immediately, put HW back on, boiler fires up again. Prior to original Head replacement, system had worked fine for 19 years ? Is there something we're missing. Any suggestions very much appreciated

JT
February 2010