I have been swimming and cycling for many years and have recently decided that i would like to compete.
I am unable to do triathlon because of the run, i have compartment syndrome in my lower legs which prevents me from running for more than 20 mins.
Cycling and swimming does not aggrevate it at all so i would therefore like to do an event that combines the two.
I have searched the net for such competitions but all the duathlons i find are run/cycle events.
Any advice would be enormously appreciated.

I would suggest you look into relay teams for tri's. Many if not all local triathlons have a relay division. Start asking around and see if you can find someone to do the run portions with you. I think you will have a hard time finding a swim and bike only event.

I have never seen an event that was only swimming and cycling. But that is very interesting. IF they have duathlons with biking/running, why not also have one for swimming/biking or swimming/running?

I've seen Aquathons...but I've only seen swim/run combos. I realize it takes some of the fun out of competing, but you can always walk the running portion. I'd also second Mike's relay suggestion. I've never seen a tri that didn't have a relay division.