Our 18month old baxi boiler keeps losing pressure and so does not work until I go up into the attic and physically turn open the tap to let more pressure in ( once or twice a week) How can I fix this please?

Perhaps this may help; Baxi Megaflo Boilers (http://baximegaflo.wordpress.com/)

Look to your outside wall adjacent to where the boiler is situated and see if it is dripping water. If it is then your prv... pressure release valve is probably needing replaced. It is a small copper pipe on the outside wall usually turned to face the wall for safety reasons.