IMO the biggest question is does the installation have "good bones" with some bodges on top that can be remediated, or does the bodgery go so deep that the only real option is a rewire.
The taped up CU worries me not because the CU itself is difficult to replace, but because it may reflect the attitude of whoever did the wiring work that is fed from said CU.
Thats my worry, how much of the system was a DIY/cost saving bodge.
Thanks for the advice everyone, ultimately you're right, the cost of remedying the extension/outbuilding CUs is fairly low, and even if it needs a complete re-wire it's a small % of the house cost.