I spent ages looking at this - it just fascinated me.
I guess it shows which way my perception's slanted, that I saw the Mercury symbol before I saw the Crucifix.
Then I noticed how the cross is emphasized, and that the circle doesn't join the cross at the top. It seems more likely to me now that it's intended as a crucifix, with a graceful halo, and diabolical horns.
I wonder what "HT YC" stands for.
Located on Royal Park Road, on the block of student flats behind the pub. It's about eighteen inches above the ground.