I’ve paid for my holiday arrangements in full. Can my travel company still request a surcharge?

Yes, as long as their booking conditions allow them to surcharge and don't say that no increases will be made after full payment is made.

Surcharging refers to your travel company changing the price of your holiday after you have booked your arrangements.

It shouldn't be confused with additional charges that you may incur for changes to your booking such as amendments or additional services.