The span is 4 metres max. Are there tables of prescribed dimensions
and spacings etc somewhere I can access?

Building regulations are available on www.odpm.gov.uk.

The old approved document Part A has tables for common wooden beam sizing,
including joists. (New part A offloads these into separate documents from
trade bodies that you have to pay for, so grab a copy now).

With 4m span, for table A3 for ceiling joists with 0.25 dead load using
SC3 grade wood and 400mm (close) joist spacing.

47x72mm = 1.27m

A 47x170mm is good for 3.96m.

Therefore, your joists are already very substantially below currently
allowed specifications. Note that the ceiling joist tables show similar
figures as for normal floors. Presumably they woke up to the fact that lofts
are used for storage, whether they liked it or not!

Christian.