However when i connect the main connector back to the mcb but leave it
switched off and carry out the same checks the open circuit
characteristic no longer exists when the 20 ring mcb are switched off.
Closed the resistance is .2 ohms but when opened the resistance is 56
ohms where i had previously had open circuit , ?????????????????? The
earth for the workshop is taken from the main house distrubution earth

Insulation resistance checks must be done with the circuit to be tested
isolated from any circuits that may have devices attached. An off MCB is NOT
isolation. This means that all appliances, hi fis, RCDs etc. must be removed
before the test.

With the MCB off, it doesn't disconnect the neutral. There will be loads of
appliances in the house (presumably still plugged in?) that will provide
various connections between neutral and earth. They're current being fried
with 500V DC between neutral and earth every time you attempt a test. That
assuming you even isolated the house from the mains supply. If you didn't
there will be a connection between neutral and earth through your TT
earthing, or the supplier's TN earthing.

Christian.