Combi stops firing for hot water when cold tap turned on?

Hi

I have a Worcester Bosch Thermotechnik combi - on the boiler casing is a Bosch-stamped cert saying it is a 28LE RSF which it also says on the front panel. On the user instructions sticker inside removable casing and the manual itself it says 28CDi RSF.

Anyway it's had a few issues over the last couple of years (new PCB and clock) but has been working fine for about a year. I have normal pressure on the front dial (about 1bar).

The problem has just started happening yesterday where you run the hot tap and boiler fires and provides hot water but as soon as you turn on the cold tap, the boiler stops firing.

Today we have to open the tap in the kitchen sink right next to the boiler almost all the way to get it to fire, but hot taps on longer pipe run (same ground floor level) in bathroom will not cause the boiler to fire at all.

Also, even with the kitchen hot tap on full if you open the hot or cold elsewhere the boiler stops firing. Central heating is fine.

So it seems it is some kind of flow or pressure sensing issue inside the boiler? I probably need an engineer but would like to know what it is likely to be so I am informed, or if it is a simple thing I can adjust safely I would do it.. Can anyone advise? Thanks very much..