Plumber one radiator, a bathroom and shower mixer and 3 sets of taps. One in ensuite, one in main bathroom and one in downstairs toilet. Found a TPR 15 reliance water control cut out lying on the floor after he left. Should this have been replaced?

I have an oil driven worcester heat slave combi boiler. No water tank anywhere.. The pressure in the shower he fitted a Mira Coda Pro EV Exposed Thermostatic Valve Mixer... is poor. The hot pipe behind the mixer is extremely hot and is leaking. The radiator in the he fitted and two of the taps leak too. One of the cold taps has grit coming out of it! Help. All advice welcome. Do I need the pressure relief Valve replaced. Where would it have come from.