after a recent oil cooler change, I have flushed and reflushed the water cooling system, using both tap water and a flushing agent. But there is still loads of residue in the water topup bottle. Is there a knack or am I just doing it wrong following the haynes manual?

The oily residue isn't just in your expansion tank - it's covering the walls of your whole cooling system.  You'll never get all the crap out with water or stuff like speedflush after an oil cooler failure.  As mark suggests, I put in 5 litres of water soluble engine and parts degreaser and ran it for a good 15 minutes.  During this, I also carefully loosened the filler cap slightly which causes the mixture to bubble up in the expansion tank and give it a nice vigorous clean.

After this, many flushes with clean water will be needed to get it all out as you can't drain the whole system on the omega, so really you slowly dilute everything out rather than flushing it out.

Liam

gunk is a product that comes to mind, i think its about £6.00 from halfords for 1 litre

gunk is a bit dear if you need 5 litres of it. Thanks will try machine mart.