Broadback wrote:
Recently I have had loft insulation installed, the "surveyor" maintained
that even though there was no obvious form of ventilation it was not
needed because the roof felt was a breathable membrane, looks like
ordinary roof felt to me.

Do you have other reasons to doubt the surveyor?

Anyway 2 questions, when is the best time to check for condensation, in
warm damp weather, cold, or when?

When the roof is cold enough for condensation to form, I found it worse
in the winter.

Second if I need ventilation I intend to put through air bricks at
either end of the gables, does it matter how high they are placed?
TIA

As low as possible but I doubt a few air bricks are sufficient. You
need continuous eaves vents (or equivalent) so that the air enters at a
low level and flows across the roof space.

See
http://www.stockton.gov.uk/resources/environment/BldgControl/BCGuideNotes/condroof.pdf#search=%22roof%20ventilation%20site%3 Agov.uk%22
and many other such pages.

MBQ