Hi Mark
If the IS is of use then it's a really good lens, but the problem is that IS is not very effective when using it at macro distances. The non IS 100mm is just as good optically and will save you around £350, if you choose to spend this money on a MT24 - EX macro flash you would have a much more effective macro kit than just the 100mm IS L.
Macro flash is a revelation, once you've used it you'll wonder how you ever did without it, images which were impossible to capture suddenly become frozen in sharp focus when you can set the shutter speed to 1/250th sec. You can set the aperture you want, and know that you will get perfect exposure with every shot.
I'd avoid the temptation to buy the MT- 14EX ring flash as you are not able to angle the flash heads or change the output on each head individually. If you do decide to buy the MP-e 65mm 1 - 5x macro lens you will appreciate the way that the flash doesn't get in the way and you can angle the head for optimal illumination.
People are selling the flash brand new on Ebay for around £500, someone has a lightly used one
here I think I'd make a best offer of around £475 see how that goes.