Hot water but no Central heating?

get a plumber in

1 check the thermostat does it make a "click" at about 18 just turn it clockwise and anticlockwise and you should hear a click

2 if it does look for your pump. There should be a largish silver box with three pipes going to it, two one side and one the other. This is a Motorised Valve. There is a small lever which will divert hot water to the radiators.

3 you may need a new "Motorised Valve" ( drain down system so could take a few hours) or thermostat (should only take 30 minutes) so check prices for them before plumber comes so you can be sure they are not ripping you off.

it sounds to me that when they changed those parts they didnt bleed out the system. you have air trapped in the rads open the bleeder valves and let the air out till water comes and they should work when you try bleeding put the zone valve on manual