Thank you I’ll get my garage to do that as I don’t want to make it worseOk sounds like you have restricted water flow. If the heat goes slowly from hot to cold this is another indicator.
You could have an air lock in the heater matrix, a kinked pipe, a partially blocked matrix or failing duovalves (if yours has them).
Start with the cheap easy things first - try reverse flushing the heater matrix and bleeding the air out of the heater matrix. The way I bleed the heater is to get some temperature (pressure) into the cooling system, then loosen the connection to the heater matrix where it comes out the firewall (usually duovalve if fitted) then very carefully slide a thin screwdriver down between the hose and the pipe to allow the air to bleed out. Be very careful as you are dealing with hot water under pressure!!!
Most cars don't require bleeding but for some reason I've had a few that do.