/If you have a router it is possible to produce a counterbore underneath using the router from above. Use a two flute cutter where the cutting edges are wider than the the tools shaft. Adjust the cutter so that the top of the fluting is at the distance from the router base is the depth of the counter bore. This might mean you will have to hold the tool in the collet protruding more than the ideal. Using the tool shaft like a guide bush will enable you to move the tool around the existing hole to produce a counterbore. Care would need to be taken not to exert too much pressure whilst cutting as this method relies on having the cutter protruding probably beyond the ideal. /q

Well yeah I follow you but the distances are too big for anything I've ever seen router bit wise - 45mm thick worktop...
Thought abt a forstner bit but I reckon I'd probably end up with a non-flat face on the inner counterbore against which I'd be trying to secure backnuts...

Also any counter bore would have to be big enough to allow a spanner (box I spose, that I don't have) to access the backnuts...

Cheers
Jim K