As RichieST posted above, it is the pedal box bracket creaking against the bulkhead. (firewall)

If you remove the lower part of the dash, and look at the pedal assembly you will see a number of 13mm nuts that secure the pedal box to the bulkhead, loosen all these off so you can move the pedal box bracket away from the bulkhead slightly, get some grease behind there then tighten all the bolts up. Usually cures them.