Flat roof

OK so I am cheating as I don't like heights and will get the job done for me but I would still like some advice.
I have a large rear flat roof loft extension (roof circa 22 foot by 18 foot) which was constructed in 1980 making my semi detached bungalow into a two storey building. It was reroofed about 7-8 years ago and is currently in good condition. I had left it too long and got leakage so the work involved replacement of boards, insulation etc. I am anxious not to leave it too long again and as I have insurance policies due to mature within the next year am adding recovering the roof to the list of projects.
The roof is currently covered in bitumen felt which I understand to have a life of 10-15 years. I note that firms installing fibreglass flat roofs talk of a 25 year life. As I am 57 I am attracted to the prospect of not having to bother with flat roof repairs again in my life!
My questions are :-
Are fibre glass roofs as durable and problem free as claimed? Are there any downsides?
What would be a realistic budget cost - I live on the fringes of Norwich?
National company or local roofing contractor?
There is a very slight fall on the roof and to date I have only had problems when the felt failed rather than from water creep.
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