This looks like Formitopsis pinicola, or possibly Heterbasidium annosum, but it was growing on what looks like Birch, which is not a host species for either.
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Some fungi are very poisonous so a mistaken ID could have serious consequences.
No interactions present.
This is Fomitopsis pinicola as you suggest. This species has been recorded fruting on Alder & Birch from Hampshire in 2008. Also on Oak from northern counties.
Thanks for the determination Andy. I thought it was F.pinicola, but then convinced myself it couldn't be as it was growing on broadleaf wood not coniferous. So it's very interesting to find out that it does colonise broadleaves.
Lat/Lng: 50.9977, -1.3456
OS grid ref: SU460222