You're probably right Charmion. But the young lad sorted it (He's called George, and such a nice chap) He put mastic into the gap, and is going to board over the spaces in the small breeze-block stack too, which will hermetically seal the overhead roof and completely protect the boiler.
We laughed and said we'll keep changing its nappies (ie the wet towels on the top) until he comes back with the materials for boarding up the gaps.
He told us to put the heating on, it would dry out any damp in the boiler casing. So we're toasty warm now. Good thing, as it's horrible outside. Flooding almost blocked the road over to Fakenham (supermarket) and loads of tree branches down, and shifted to the side awaiting collection.