I'm no expert but it looks like a leaf to me.

Funguy cut the daft comments... Snoop is the whatever it is raised up from the leaf - it appears to be. Don't think I've seen anything like this before so I someone does know - perhaps Scrumpyg?

Had a quick look at the BBC gardening website under powdery mildew. I know it doesn't sound like it, but if you read down it says it can cause yellow spots, so could be! Have you cut into the 'warts' to see if there is anything inside the leaf living on the sap?

http://www.bbc.co.uk/gardening/advice/pests_and_diseases/identifier.shtml?powdery_mildew

It looks like a leaf gall, usually caused by mites.
Not usual on cabbages as such, usually appears on leaves of trees. Any nearby trees showing similar problems?
Fairy liquid won't cause it :)

Oh, leave Funguy's alone, he's only being happy. Probably had a beer!
The leaf crustiness looks like it could be white blister, peculiar to brassicas. Grows in humid or moist air, and treatment is to remove affected leaves, avoid overhead watering etc etc. Nothing's safe these days is it? Even the stalwarts of the veg patch

Sorry guys,its been a tough day. My missus has packed her bags and gone - because of my fetish with touching pasta. I'm feeling canneloni right now.

Anyway good luck with veggies.

Funguy - if your wife has upped and left you I a truly sorry...

I think it was another joke MG...
and I did laugh about the advice to have a cuppa when the hollyhock flowers die. (Doesn't take much...)