Hello,

I have a Boardman Comp (Aluminium frame) size large. It has a Truvativ GXP bottom bracket with Truvativ Elita cranks.

When I put the bottom bracket under a lot of strain (going up hills, standing on the pedals) I hear a lot of creaking. Is this the frame flexing or something else. I can't quite figure out what the noise is? Can it be reduced somehow?

Nevica

Assuming your bb and cranks are spec'd for on another I would do the following [ if you can't do it, have your LBS do it ]:
#1 Look at your farme and make sure there are no cracks in it near the bb welds
#2 Remove your cranks and then remove your bb.
#3 Clean and grease your frame's bb shell.
#4 Put teflon/plumbers tape on your bb cups, then reinstall them [ tighten to spec ].
#5 Reinsatll your cranks [ tighten to spec ]. Also make sure that all your chainring bolts are tight.
#6 Remove your pedals, grease the threads and reinstall your pedals.

How do I know if bb and crank are speced for each other?

So it's not normal for the BB to creak under pressure.

Also add to above: Check your cleats, if you have clipless.

Nope its definitely not the cleats

Look at SRAM's website, as Truvativ is now SRAM or vice versa.
Anyway their bb's are specific to certain model cranksets, so make sure the combo you have is the correct one.
If the SRAM website/tech folks say you've got the correct combo, then try my suggestion for eliminating the squeak.
Good luck.