Lozzo

1) The front wheel came out and went back in to do the tyre replacement - they incorrectly fitted the speedo drive when refitting (easily done but that's no excuse, they are a Suzuki main dealer so they should be aware of this) so they are obliged to supply and fit a new one.
2) They MOT'd a bike with no horn working - I'd point this fact out to them when they start getting shirty. VOSA may like to take a little look at how they conduct MOTs.
3) The chain wasn't adjusted correctly, if they adjusted it at all - that's three things they got wrong or neglected to do to your satisfaction. Btw, chain tension is a part of the MOT test, so that's two MOT fail points you have as leverage.

Get back in there and ask again that they think again about rectifying the problems they caused with your bike or you'll be forced to take the matter further.

You presumably paid a lot of money for a platinum plus service and as such expect all the work you contracted them to do to be done to a satisfactory standard. Obviously they haven't done this so you want them to remedy any faults and for a different technician to check over the bike to ensure that whoever did the work completed the other items on the schedule to a satisfactory standard. the fact your bike has 50K miles is irrelevant, fact is the horn worked before the service and somewhere along the line they managed to disconnect it - the speedo worked before the service and somehow they managed to break the speedo drive when refitting the front wheel.