First of all Wayne, don't start thinking how expensive it's going to sound - I would imagine they have spent in excess of £ 10k for the new carpets.
I usually use Prochems Flouroseal, other brands are available
Pricewise, I would - and have, with this size of job - charge £2 per m2, you can justify a big part of this price simply by the cost of the product, the rest is easily justifiable in labour cost.
Pre-spray the carpet with a weak solution of fib/fab, then spray the protector east-west then north-south, then brush in and groom using a CLEAN pile brush.