you are stuck between a rock and a hard place if you want someone else to paint it... as said it needs to be off the bike. but then you won't be able to take the bike to the painters to make sure that the colour is a good match to the other bits around it.
i know you say you've bought paint from rs but the engine and casings are not lacquered! so have you bought pots or aerosols? (i've assumed you're trying to match the rest of the engine, not colour contrast your casing.)
assuming you have somewhere suitable to paint this piece you'll be needing wet'n'dry - say some 240 (if it's bad) then 400 to smooth your casing. use plenty of water. then a skim of p38 if necessary, then more sanding. then an aerosol (assuming you don't have a compresser and gun). i strongly suggest painting a sample of card (or similar) with the paint you've purchased to see what kind of match it is. there's more than one good colour match available, personally i use simoniz aluminium engine silver, £5 from ecp. it's a good match and weather and heat resistant.
lastly you'll need a gasket for re-fitting. you might get away with re-using the current gasket, but if you bolt it back together and it leaks you'll regret not spending £4 on a new one. you can either take the cover off with the bike on the side stand and suffer a small amount of oil loss or the oil will have to be drained to allow clean removal, then you may as well change the filter while you're at it.
very lastly i could paint it and post it back to you, not for much but not for free either.
alternatively take the bike as it is to a painter and be prepared to wait for it and write a cheque when you pick it up.
hth. ramble over.
is it clean enough?