dog-man wrote:
Any roofers here who could etimate (very roughly) what I could expect
to pay to have the roofing felt on my average size 3 bed house roof
renewed?

The felt has become dry and brittle and is letting rain in. I have done
temporary repairs with filler foam as I was renewing the ceiling
plaster boards, so had access to it. No attic space in my house.

I will be getting some quotes, but was just wondering if anyone here
had an idea what it might cost me.

If this is a pitched roof, then there should be little ingress through
the felt, and your problem may be with the tiles - ridges or valley.

Its very rare for felt only to require replacement on a pitched roof.

Felt on pitched roofs will normally only allow water ingress where it
is torn, or if rotted at the eaves gutter . It does not deteriorate
like flat roof felt. A patch repair at any tear or hole will suffice.

dg