You will need both planning permission and building regulations
approval if the conservatory is attached to the house and it
exceeds 10% of the existing floor area.

Apparently, though, the 10% applies within a range from 50m3 to 115m3, so
the minimum permitted development rights are 50m3, however small the house.

A very rough estimate of volume is 3 x 3.5 x 3m, which gives 31.5m3, well
within the limits and enough left over for a lean-to over the side of the
kitchen. The house was built in 1909 and there has been no previous
extensions, just a garden shed, not attached in excess of 5m away from where
the conservatory would end.

For building regulations, different rules apply, based on a 30m2 floor area.
Total area will be about 10.5m2.

So I can't see why I'd need Planning Permission or Building Regulations
approval.

Christian.