timber block supports for rafters

[Gavin A]

I think installing an extra rafter and fixing it to the wall plate and bid mouthing it at the ridge is impossible in most the roof spaces i have been in

doesn't need to be - you're strenghtening the existing rafter to ensure it can cope with the extra point loading in the middle of the spans, not strengthening the actual fixing to to the wall.

if it's got a purlin in the middle you can just cut the extra rafter in the middle and bolt each side separately to the rafter above and below the purlin.

eta - usually, unless you structural engineer says different of course.