I could not get hold of a vice to try myself so I relented and took it down to the LBS to get it sorted. Seconds later the wheel was free of its cassette after the bloke there used the same technique I was going to use. It still took a lot of force to move it though and he said it was probably way over-tightened when it was assembled. The new cassette was put on and tightened securely but not overly so. Hopefully the next time the cassette is replaced (2-3000 miles later) it will come off with relative ease.
Now that it is done and dusted a new chain (KMC) has been put on with aid of the re-usable QR link supplied with the chain, so no more chain splitting tools to assemble the chain. It was the same length as the old chain when laid down on the ground so I just placed it on without any adjustment. Although in hindsight the extra QR link may have added some extra length so maybe a link or two need removing to get it spot on. All seems to be running fine ATM with no issues with changing gear.
The rest of the bike has been thoroughly cleaned whilst it was free of the wheel and chain so it looks fairly nice now, aside from the various scratches on the paint, ripped bar tape and rust on assorted nuts and bolts

. Hard to believe the bike's only 13 months old!
New tyres (Schwalbe Marathon "Greenguard") on the way from
Evans' so will fit them later in the week once they arrive. They are replacing the original Conti Sport Contacts (now only on the front) and a Duro-Branded tyre on the rear. They should last me a couple of thousand miles without too much trouble. They aren't Pluses but I didn't want the extra weight or harsher ride that I have heard some people commenting on.