Finedon Slabs - the parapet. © Wingnut
The "rock" is actually concrete - the remains of an ironstone crushing plant whose cracked and weathered remains now provide Northamptonshire's only entry in a climbing guide. A perfect opportunity to use the word "esoteric". [Liz Asquith]
[OS reference is approximate; better one welcomed - UKC Ed.]
From the junction of the A6 and A510 at Finedon, turn into Orchard Rd, and then into St mary's Avenue. About 100m down the lane, turn left onto a footpath, and follow this for another 100m until the crag is reached.
It's in the "Pocket Park", which is a wooded ex-quarry. The flat top of the "slab" is clearly visible in the aerial photo.
guidebook => "Leicestershire climbs" - Cordee