Thanks @Marvin
More complex than I though, good job I will be using an architect / planner when the time comes!
I see there is a section on "Side wall extensions extending beyond rear walls" (p20). If I understand this correctly, then a side extension that is not attached to the original rear wall can extend beyond the rear wall of the house, and the bit that does is to the same limitations as a rear extension. At the moment the side extension is double-storey between the front and rear of the original house, with the small (1m ish) part, that extends beyond the original wall, being single storey.
From what I am seeing, I could leave the conservatory as it is, and extend the existing side extension further to the rear. If I understand what it is saying on page 21, then even though this part would be single-storey it would be governed under the rules of a double-storey extension as it forms part of the overall side extension which is principally double-storey in nature. In this case it would be limited to 3m.