We have a room sealed oil boiler and when it was moved the only place it could go involved the flue outside too near to the utility room window. The solution agreed with building control was to seal the window so it could not be opened and allow fumes in the room from the flue. The agreed method of sealing the window was to lock it shut with the key and then remove the handle and blank over the three holes (two for the screws for the handle, one for the handle mechanism). This has all worked fine except we can't find a blanking plate of any sort to cover up the holes in white upvc. I don't really want a self-adhesive one as getting it off if the window needed to be maintained (with the boiler switched off) could be tricky. I've tried a variety of places but nowhere can help with a blanking plate and seem astonished that building control weren't happy with the window simply being locked. However, as all of our window keys are the same, they were concerned that somebody could simply get a key from another window and open up the window and kill themselves with the carbon monoxide and we'd rather be safe than sorry. Can anybody think of a solution for me?