Unless the chain is slipping, will it change the experience? Are they very different parts your fitting? I would ride a little and see how you get on with it.
Ultegra cassettes are silly prices IMHO at the moment, so I'd stick with 105 and save the £30/40. The weight saving is only something like 30 grams.
Unless your chain is slipping, I would ensure it's cleaned and lubed so running smoothly / quietly and sit on the money. I'd agree with @AndyCr15 - unless your existing drivetrain sounds like a cement mixer, it's not going to improve a ride.
Cheers both.
Having not actually ridden it yet, i'm not sure. The bike guy suggested the main issue was that the chain struggled to move out of the smallest cog at the back and so i would ofter need to go up 2 and then down 1 to get that gear. He mentioned this was a sign of the chain being an issue (along with a huge amount of play in the chain). He also gave me some chain link things incase the chain snapped on an outing which can't be a good sign!
I think you're right, i'm sure many bikes will be in worse condition than this one and still being ridden. I think i'll try and run at dinner and then get on the bike tonight. Trying to decide whether to buy a phone mount for the stem for any routing directions or to mount my Garmin Fenix on there as it'd also give other info.