No it's not too bad Dan but it's time consuming. I probably spent a couple of days researching the engine loom on my car and then another day drawing out a wiring diagram and doing pin to pins.http://item.mobileweb.ebay.co.uk/viewitem?itemId=300371984903&index=1&nav=SEARCH&nid=24144169187
That's the semi assembled loom...
Is it a major task to do something like that yourself?
You would need to understand what needs to be controlled by the Omex and what needs to be run by the std ECU but thats pretty simple. I recon it would take you a couple of days to do if you wanted it to be nice and neat. From the price you say Paul has quoted it sounds like you are paying for around 7 hours labour which is pretty reasonable in my opinion for an engine loom (My very simple one cost me 13 hours labour originally)
Edit: beaten to it by Chip