There was a similar request for help from Therion last year, have you used an accurate grid reference? would it be worth putting your grid reference into google earth or an online grid reference finder.
If it's coming out in the same (inaccurate) spot, it might be worth changing the coordinates to where the online grid reference finder thinks it should be?
https://ukcaving.com/board/index.php?topic=21946.0
"I can though see one big error. You need to use 5 digits on your grid ref. Using only 4 will put you cave miles away from where you want it. Also to make it fit GE you may have to fiddle around with the last numbers to place your entrance exactly where you want it. It seems GE images can be a bit distorted so quite often when you do surface surveys of roads, dry stream beds, and entrance to entrance etc they sometimes do not fit too well.
As for the code in your Th file I use this and it works.
survey ChurchPot
centreline
cs OSGB:SE
station 1.0 "main ent" entrance
fix 1.0 09959 76033 217
declination -1.4 degrees"