In reply to Southern Man:
"There's a 15 foot outcrop of conglomerate near Hertford"
Is this the pile of sand/pebbles in Waterford marsh? The nature reserve that was once a quarry?
When I first saw it from a distance it looked climbable; I thought I had stumbled on some overhanging bouldering right on my doorstep, then I tried climbing on it and nearly pulled the whole thing down.
There are a few brick railway bridges in the same area that would make great DIY climbing walls like a mini version of the tunnel in Barcelona.