How do i pressurise a Worcester Bosch 400?

I have lost pressure in my Worcester Bosch Highflow 400 RSF. All possible solutions talk of either using a key which doesn't seem to be right for this boiler or using a flexi filling loop.
I can't locate the loop as the boiler is encased in a unit in a utility room, i can only access from the front where nothing is apparent.
Looking for any possible solutions, no heating and it's getting cold!
Cheers