right things to check
1st make sure the fill pipe is shut and remove it ( the hoze part ) and see if any water is leaking from it, if that is ok

2nd when cold make sure the level is on about 1

3rd you will have an expanshion vessel ( built in to boiler ) this has a tyre valve on top of it check the pressure on this as if it is 0 you will not have any room for the system to expand in to, if so look at your boiler manual to see what to pump it up to ( use a bike pump ) probley about 0.5 bar

hope this helps