having a wood burner fitted soon, the old gas fire has been removed and capped, I'm looking at increasing the hole by two bricks but there is a lintel fitted. can i move it up to the level i require.

Yes the lintol can be moved up, its probably supporting 100mm bed at the moment, depending on angle of the gather you may need to add another lintol to the side. Check with your Hetas installer for all minimum space around and above the stove, theres a lot of heat coming from a wood burner.

If there is a gather fitted the lintel would need to be fitted above this and possibly two smaller lintels on either side to support masonary above. It could be that there is non structural masonary which has been used to create the throat, if thats the case then a lintel can be put in higher up and the non structural masonary can be removed leaving a larger fireplace opening.