When I record in audio, my keyboard tempo is slightly out of sync with the project tempo. So, if I record anything direct from the keyboard at, say, 120bpm, it is impossible to add audio loops as they eventually lose sync. Now, I seem to remember someone posting a tip about how you can determine the actual tempo of a project by highlighting a number of bars within an individual track. This might give the answer 119.7bpm, and then you change the project tempo, and - hey presto! - loops will work. Trouble is, I can neither remember the details nor locate the thread. Can anyone please advise further? Thanks.

Don't think I can answer properly but will ask:  is there a way your hardware interface presumably running at the project tempo (record/playback) can send MIDI time codes or syncs to your keyboard and its interface?
   This temporal difference might be an artifact from the different, separate oscillator frequencies used by 1) your soundcard or hardware interface and 2) by your keyboard.  I expect the tempo delta will increase the longer the song / score you are composing or performing.

Pacofat - you are clearly more knowledgeable than me. I base this on the fact that I don't understand a word of what you just wrote.
I have an audio lead from the keyboard to an Edirol UA4FX external soundcard, which is linked to my PC. I record audio directly from the keyboard into Sonar X1 Producer. Even if both keyboard and software are set to the same tempo, there is a definite parting of the ways as the song progresses. Slight, but significant. And I really need to put it right.

I haven't done this so YMMV, but I'm assuming you're having the keyboard sequencer play music given your explanation. The project's tempo is what you see in the tempo indicator at the top of the command bar. There isn't a secret place to grab the tempo that will somehow be different than the project tempo that I know of. There is a way, however, to have Sonar synchronize to the keyboard's internal clock. Check out Synchronization in the help docs. Here's a snippet of the Help doc that may apply to you. I hope this helps get you going.
 

MIDI Sync

MIDI Sync is not the same as MIDI Time Code. MIDI Sync is a type of synchronization that is usually used to synchronize drum machines, desk top hardware sequencers, and internal sequencers that are in keyboards. The MIDI Sync format uses a timing resolution of 24 Parts Per Quarter Note and Song Position Pointer messages. When using MIDI Sync one of the units must be designated as the clock source.

Options | Project | Clock

Set the clock source to MIDI Sync.
Follow the directions from your drum machine or hardware sequencer to have it transmit MIDI Sync. When you start Playback on Cakewalk, a MIDI Sync dialog box will indicate that Cakewalk is waiting for MIDI Sync. Then start your drum machine or external sequencer and Cakewalk will begin to play.

If playing in real time the clock for the tempo playing your keyboard is your brain. The Sonar clock for audio should be set to your audio interface, and the tempo itself is generated by Sonar from that clock. If the tempo you are playing to record audio is not matching the tempo in Sonar, then your playing is out of tempo--no clock issue exists. Now if you want to match a loop of your recorded audio to a fixed tempo (beats per minute) you need to stretch or shrink the clip that represents that audio to the real time length of the proper number of beats at the Sonar tempo, using an audio stretch algorithm like slip stretching or process fit to time. 
 
But you are saying you are playing from your keyboard at at tempo of 120, which implies that you are not actually playing a keyboard but a sequencer built in to the keyboard. The simple kludge to fix a limited number of loops is the same in this case as for playing live out of tempo. If you need to match the keyboard sequencer to the Sonar sequencer for continuous playing however you will need to set up one to follow the control of the other. Why do you need to use two sequencers simultaneously?