Rover 25 after well, after 2 mins stops . starts again after 30 seconds and ok for rest of the day

This engine was never designed to have a replacement headgasket fitted, it was an after thought and therefore can be a dreadfull job that many garages will not do.
I would take the car back and as the garage to check the diagnostics. I would do a coolant test under pressure and another head test to see if water is seeping in when the engine is cold. Once hot it will gave expanded and will be ok.
Remove the plugs and crank engine, see if any water comes out.
If so, you could try a very good "radweld" type material or "realsteel" that may work.
The reason you wait to restart is to dry out the spark plugs.
**** yes, i have rebuild many of these and wish i hadn,t***

Never heard of ANY engine " This engine was never designed to have a replacement headgasket fitted " that this applies to !!! Wet plugs dont dry out themselves in 30 secs , especially with a cold engine . The head problem was " cured " by the modified kit brought out years back .Was your head tested and skimmed prior to refitting? Only use the suggested additives as a last resort if not prepared to sort correctly as they may knacker / block your cooling system .Try changing engine coolant temp sensor , simple and cheap job .