In reply to Jimbo C:
>Do you know if those are dense concrete blocks, or the lightweight ones?
Fortunately they are dense ones.
> On the other hand, if the drilling is likely to 'pop off' some plasterboard / plaster, you might be as well to cut some out neat strips of it to allow face fixing of some straps and then simply fill over them.
I think I might go for a mixture of both. The front of the house is a bathroom between two bedrooms. Two holes to fix four of the rafters above the bathroom and keep my fingers crossed it doesn't affect the tiling. The other issue with the bedrooms is they both have 10ft windows with just a concrete lintel and the capping blocks above, so for several rafters the best I can do is attach them through the blocks to the lintel ( I think any metal work should be in the bottom part of the lintel?) I'm thinking if I take your advice and attach straps to the rafters above the corners of the bedrooms. I'm thinking I could also use something similar to hold the lintel down as well. Bits of walls that are hidden by wardrobes and or curtains are the sort that Mrs J allows me to plaster. Using some straps on the gable ends would be easy to do as well.
Thanks Rob