[brucek]

i'd like to measure my room response so i can either use corrective EQ in my music software or buy acoustic panels to achieve the desired flat response!

As I'm sure you know, above the subwoofer range, treatment is the desired route to follow. Perhaps some low Q filters can be used at higher frequencies, but room treatment would usually be advised.

I see that i need the SPL meter to measure the 75db, as this seems quite hard to come by in the UK, other than ebay, is there another way to do this or a way to guess?

Yeah, you can guess at a reasonable measurement level when you get to that part in REW setup (where it tells you to adjust your receiver to get 75dB at the listening position with an SPL meter).
After its set at a reasonable level, you click the REW SPL calibrate routine (which simply tells REW what your SPL meter is reading). You set that to 75dB. Yeah, maybe it's actually 79dB , but it is not too important.

I have succesfully done a soundcard Cal, but was wondering if i need to include my mixer (preamp) in these tests, or to wait until i use the mic.

Some people include their mic preamp into the soundcard cal loopback when they create the soundcard cal file. This then compensates for its shortfalls when measuring. If the mixer is of any decent quality though, it won't have that much effect. If you want to see the mixers actual response to make that decision, do a normal cable loopback soundcard cal file creation, then load that soundcard file into REW and then make a measure of the mixer itself (using the right channel of REW and cables to and from the mixer from the soundcard line-out and line-in). You have to mess with the levels a bit, but it will take a frequency response measure of the mixer and then you can see what it's like, and then decide if you need it in the soundcard cal file.
The mic itself, of course, is compensated by the mic calibration file (which you load manually).

I guess your listening position is the mixer chair position?

brucek