From CAP 733 page 31...
a) Article 9A of the ANO 2000; for those limitations of a general nature that are
applicable to all classes of aircraft operating on a Permit to Fly. The principal
limitation is that Permit to Fly aircraft are restricted to flight by day and in
accordance with Visual Flight Rules (VFR) unless the prior permission of the CAA
has been obtained. Permission for flights under Instrument Flight Rules (IFR) will
only be given in exceptional circumstances, such as, to meet a long distance ferry
requirement. In such a situation the alleviation would be subject to agreement on
aircraft instrumentation and pilot qualifications.
b) The individual aircraft’s Permit to Fly; for those limitations which are applicable to
either that individual aircraft or that class of aircraft. In particular, restrictions on
flight over congested areas will be contained in the individual Permit to Fly.
Nothing about sight of surface - so far as I know, unless there's something on the permit, that is entirely down to the individual pilot's licence limitations.
G