Thanks all for your replies. No the insulation is on the floor of the
loft. It was not that I did not believe the surveyor, just that he was
not a surveyor in the true sense, so thought I would keep an eye on it
myself. I was offered (by a different installer) the option of simple
plastic wedges that hold the roof felt away from the tiles. If I
install the vents the plan was one in both gable ends, which are at
either ends of the rather long and narrow loft. If I need to do that I
cannot complain as I had full loft insulation and cavity wall insulation
installed free!

If you have just had insulation in the loft and the underlay felt is a
breather type, This sound rather quire, as breather underlay felt is a
modern idea. Therefore why have you had to have insulation in a modern
house. Either the property is older and as been re roofed with modern
breather underlay. As regards holding the roof felt away from the tiles,
this comes natural when the lath are fixed and the tiles laid. The underlay
should sag away from the under side of the lath by 10mm to prevent, what is
known as ponding, ponding is condensation that runs and drips of the back
of the outer roof covering either slates or tiles. It as been found that
venting should also be carried out, not only to the roof space, but also
venting between the felt underlay and the roof covering.