To cut a long story short, our builder is going to take a newly but very poorly put on flat felt roof off from our utility room extension and replace it with a fibreglass roof.
He has said that he won't charge us for the additional cost as it was the (reputable, well-used!!) roofer that he got to do the work that did the crap job with the felt roof, but it feels fair to me that we cover the material cost of the fibreglass roof in addition to the one lot of labour that we already have. Unfortunately for him the builder will have to take the hit on two lots of labour.
My online searches suggest that fibreglass is approx double the cost per square metre, although that includes boarding etc which is already done.