Problems getting hot water from combi boiler?

Help!!!!

I have been having problems with my Vokera excell 80sp combi boiler, and have had an engineer out.

It all started with two leaks coming from the boiler and pressure sitting around 3 - 4, engineer came out and fixed the two leaks and advised that we need a new expansion vessel and prv fitted. and explained that we will properly still find that we will get a small leak until we get this fixed.

The engineer came out yesterday and fitted the parts but now we are experiencing our hot water going hot to warm then cold rather than staying hot, the pressure is sitting at 2, and we still have a tiny leak, came back today and had a another look and changed a couple of pipes, but that failed and will have 2 call them out again.

can anyone help