OP doesn't even say whether a detached garage.
Irrelevant.
Is it? He says the CU has a "
main incoming breaker 32A", not what the supply to it has.
The problem is that Volex don't appear to do a CU with a 32A breaker as an incomer, or even a 32A RCD, so what do we have here?
And it's not normal, is it, to buy a CU with a switch or RCD incomer which is not cabled to the N bar, or at least supplied with a length of cable, so what do we have here?
Do we have some cobbled together unit rather than a proper CU?
Do we have someone asking who has mis-described what he has because he doesn't know what he is looking at?
what to use for the 50mm long link?
Why doesn't someone who is competent to install a CU know what size cable to use given the rating of the device protecting it?
As the OP will have got so far and will (have to?) continue whatever
No, he doesn't have to continue whatever, what he has to do is to stop right now and get someone who knows what they are doing to install this.
I'd suggest that, in the absence of "blue" wire s/he should use a piece of 6mm or 10mm (as available) Y/G and sleeve it with blue.
I'd suggest that he engages an electrician.
And I would assert with utter certainty that any advice or assistance which leads to him continuing to grope his way forward is dangerous and irresponsible, and absolutely not in his best interests.