Many thanks for the info Paulus!

It don't look much like a 'christianised monolith'. The 'upright' looks like a recycled gatepost, complete with the remains of rusty hinges.
The horizontal piece looks older, but has still been squared-off by a mason at some point. One end of it looks like it's got that 'fluting' on it, like it's been weathered for a long time, but in an upright position...

I've posted a pic so you can see what it's current state is. Has it changed much since you saw the cupmarks?

Cheers
Dave