I'm not familiar with that hill but I know the area is hilly as my brother lives around there (Bathampton).
That's fast for an MTB.
I did manage 59.5mph on my road bike a couple of years ago. That was down Jubilee Way on the A2 just outside Dover. I had to slow down then as I nearly went into a gravel trap for lorries! :shock:
Porlock hill is fairly famous for being properly steep, Google it. Its a 1:4 with hairpins but it opens out at the top and eases to a 1:6 then a 1:8. That speed was on the way back down between the 1:8 and the 1:6.
My near 20 year old canti/U-brake setup really struggled to scrub of all that speed as the hill steepend and started to get twisty, it was a bit exciting. I had about 2 seconds of good brakes before they faded; 240lbs (me and bike) from 50+mph on a 1:6, fast becoming 1:4 :roll: