Roker - if the felt is old & needs replacing, then best to go that route... you could just replace it with felt again, which will last another few years... there isn't much to it (depending on the roof size!) if you are any bit DIY handy.
I've used Thompons roof seal in the past (available in B&Q, Woodies..) - found it excellent, but did give it 3 coats & top up each year... which can be brushed on over the felt...
Other options are corrugate plastic / iron, or get some sort of tiles... these are bigger jobs than just the felt...