Dealing with plaster cracks

Bit short on detail really. Generally speaking, filler shrinks if it's applied too deep in one go, and some shrink a lot, some hardly at all. And if you need a lot, bonding plaster is cheaper.

Thanks animal, I was planning filling any big gaps in the breeze blocks with cement. Then I could get some bonding plaster as you suggested to go over it when dry. I will probably end up removing about a 4" wide line of the existing plaster following the cracks around the window to make sure I fix any gappage in the breeze blocks like I found in the other bedroom under the plaster.

Cement/sand is all good. Whether you need to rip plaster off & fill gaps really depends. If it's the odd small gap & the wall has properly fitted insulation I wouldn't worry. OTOH if it's an old wall with no insulation, open cavity top & dot&dab pb, gaps would be a real problem for heat loss.

As well as filler shrinking which can be helped by not filling completely in one go there is also a problem of surrounding brickwork and plaster sucking moisture from the filler or new plaster causing hairline cracks around the edges of filled crack/hole. I always first seal the whole area to be filled with a 50:50 PVA:water mix and let it dry and often a second coating of the same and let it dry enough to become very tacky before filling.

I've had variable experiences with tubes and small tubs of pre-mixed filler and these days a powered filler where you add water is my preferred option for this type of application.