I purchased a USB power pack (new trent IMP1000) which supposedly is capable of charging a connected device at 1A charge.

The supplied cable is one of those multiple-connector cables. I'd guess, since this goes via a "pin" that D+/D- aren't connected at all.

However OOTB the N900 appears to charge at around the expected 500 mA rate

I got a spare N900 data cable and split the insulation, connecting white/green (D+/D-) together on the microUSB end (leaving the charger end open)

The phone still charges fine, but again seems to be at the 500mA rate

Any ideas? I'm going to be camping/watching f1 for 3-4 days from this thu so was hoping to have access to the higher charge rate

is there a way of querying the phone to check rate?
To set/override it (fairly safely)?

it is of course possible it's an issue with the charger, though I've not got anything else I can safely draw 1A usb from.

If cat /sys/devices/platform/musb_hdrc/charger says 1 then it's not limiting input power.

If D+/D- isn't shorted, and nothing talks usb on the other end, it'd be limited to 100mA input.