Thanks for the replies guys.
Steeping hops at 80 degrees C (176 F) for 30-60 minutes after the boil seems very common among UK homebrewers, but when I have tried it, the results have been nowhere near as good as chucking a load of hops in at flameout (or element off in my case) and chilling as quickly as possible.
@denny This is surely a function of the enormous volumes pro's have to deal with, compared to say my 5 gallon set up. When making a hoppy beer I 'bang and chill' and frequently dry-hop also.
@mabrungard I'm not sure about the science, but I don't agree that they steep hops in the whirlpool for a long period to improve extraction. My guess (and it is a guess) is that the primary function of extended whirlpool time is to have clear beer to run off into FV. My reading up on this, along with the conversations I have been having leads me to think that there are two pertinent points when it comes to late hop additions: 1. Volatile aroma oils boil off very easily at temperatures below boiling. 2. More importantly though, these oils do not boil off if they are in solution. So I would guess that by whirlpooling the wort, the oils are more easily solubilised and stay bonded into the wort. When I tried the post boil 80 C steep, the wort developed a large oily slick on the surface; presumably this was where my volatile aroma went to evaporate, and as none if it was mixed back into to wort, I'm pretty sure none of it ended up in the finished beer.
@brushvalleybrewer Very interesting observation! I have tried to find this on the BN but couldn't. Is it on Brewstrong or the Jamil Show? It's interesting (to me) that a lot of UK homebrewers seem to be doing the extended steep 'because it's what the pro's do', and in fact are encouraged to do so by pro's on homebrew forums. I suspect it's completely unnecessary on a homebrew set up, and unless they're actually whirlpooling the hops or somehow improving the solubility of the oils by mixing them into the wort, lots of homebrewers are missing out on the aroma they could/should be getting.
@DrewG Extra IBU's are certainly a consideration with hop steeps, but fairly minor in the grand scheme. Your recipe looks good (I may give it go). 20 minutes is a fairly short steep time compared to what I have heard about in the UK, and I have no doubt that it gives good results. Finding out at what point you start getting diminishing returns is probably what is most important in the debate.