Hot water issue first: Warmflow oil combi? Is there a restrictor, not sure. I thought the flow switch in these was a paddle type like the worcesters. Leave it alone. There is a non return valve but it has to be there.
You will find that combi hot water performance varies through the year. At the moment, the water coming out of the ground may only be 4 or 5 degrees, so for the boiler to heat this instantaneously requires a blistering amount of available energy.....and although these combis have a well sized heat store, which is kept very hot indeed, the plate heat exchanger is frustratingly small.
Also, flow rate. Open a big bath tap which offers little in the way of hydraulic resistance and as much water as possible is rushing through the small heat exchanger (silvery box with 4 pipes attached to it) so it struggles to transfer the energy.
Do one of two things........ dont open the taps as far, or if there is an isolating valve of some manner on the cold water going in to the boiler, shut it off a little and see hot the hot water temp gradually improves. You sacrifice a bit of flow rate for improved temp gain.
General: It's unlikely you need a new boiler, just because this one is tripping out on the high limit stat. A vial type high limit stat and the relay it attaches to is about £20.
It is likely that the other stat (vial type) in the top of the boiler is knackered, so not shutting the boiler down in a timely manner so the boiler temp is getting to high and the high limit stat is shutting things off to maintain safety.
Again, £20 for a vial type and relay. Both of these are rudimentary repairs. Ring warmflow, however old the boiler is and they will put you in touch with a local warmflow approved engineer.
Wiring: If it is an external time clock it could be wired incorrectly but this would not affect the stats as the wiring in these goes to one terminal block. You can't over ride the internal stats.
Pressure; There is a low pressure cut out in these at 0.5 bar, so any cold pressure above that and you will have normal function.
If any of this helps, you're welcome. Any questions, feel free to come back to me.