Sounds more like a job for a handyman or DIYer to be honest. In the current SSIA fueled climate it may be difficult to find a builder or contractor who'll take this on. Bord Gais are unlikely to be of much help, if they arrange a contractor, the cost will probably be excessive.
Leo
All you will need is a hadyman, 1 hours work some evening as a bit of a foxer no hard or complicated work invoved at all just a drill and make sure he leaves a tidy finish!
If the original vent was sleeved in pvc pipework and this was filled with concrete it should be relatively easy to remove the concrete. Either way this is a small job for a competent handy person. Additional insulation in the loft will actually help to insulate your rooms from excessive solar heat gain via the roof space. My own attic is very well insulated but first floor bedrooms are still intolerably warm during this weather, regardless of windows open and vents.
DIY old chap, go to your local hire shop get a small kango , give it a few belts of that, if its pvc lined it'll pop out in p**sing time, if you have another half valuable brownie points gained.