Recent drop in hot water pressure

Have you tried re-filling the radiators and bleeding them? There could be a lot of air still in the system which needs repeated bleeding and filling - can happen if not filled and bleed in the correct order Highly unusual but could the plumber forgot to have re-fill/test the system after fitting the new radiators?

If your boiler pressure is at 1.5 bar sounds like your pressure was normally high and plumber set it at 1.5bar whe your system is cold 1.5bar is the correct pressure to be at heat increases pressure whem heating on will rise but go back down to 1.5bar sounds like plumber took off rads and put your pressure at correct pressure

Are you saying the hot water from your taps, shower etc has dropped pressure of your heating system pressure has dropped as these are two serperate systems off one boiler effectively

Hi no, sounds to me like you could have 1 or 2 options 1. Theres some sludge got stuck in the filter or plate heat exchanger or 2. The cold feed to the boiler (sometimes with integrated filling loop) hasn't been put back into the correct position. Some photos of under the boiler would help identify this. Hope this helps