Not much to add to Mary's sane and as ever sensible advice, except some
things you might want to bear in mind:
Wasps don't return to old nests, but the same things that attracted
them to your loft in the first place are likely to continue attracting
them.
Wasps aren't particularly destructive, but they do use 'raw' materials
to construct their nests, quite often these will come from outside, but
it's not unheard of for them to eat wood (joists) or cardboard from
within the loft space for this purpose.

Basically, there's no need to rush/stress/panic, but do try and find
out how they made it in and then block their entry so they don't become
an annual little 'visitor'.

Seri

P.S. don't fall for the misconception that wasps are bad, they're
actually quite nice little buggers (unless they sting you).