All went well until taking off the oil filter, I couldnt get the filter bit off the cap, turns round but doesnt seem to unscrew or anything. So I googled it and end up on Merc forum, sure enough, other peeps have been perplexed. Turns out you just give it a tug and it pops off! It did.
Seven and half litres of oil it takes, wont be doing it again too soon then. Old oil looked okay too, probably thanks to the previous owner who used to look after it properly.
Anyway, many thanks to the Internet, Google and the MB Forum.