My boss messed up by sending me on a controversial training course when told not to by HR and hid use of internal budgets. His boss is v unhappy and heard i might raise it with union and others as i needed to see doctor afterwards.

They have said its not my fault and are sorry so have offered me 3 months garden leave and a lump sum so all in all totals £11,000. I waive my rights to raising it with union and cant work for firm again and get a clean clear reference.

What should i do and why are they doing this. Can i get more money and what should i be careful off. They are going to get me free legal advice too

Why can't you work for the firm again? After all it was your boss that sent you on the course it should be him getting the push not you.

See legal advice ASAP

Think of it as a holiday AND a chance to change direction at YOUR pace. go for it.

Without knowing the exact details of your problem it's difficult to assess what course of action you should take. It is , however, evident that you are a pawn in a game that was played at a higher level, and that now you are the sacrificial lamb.
As this for this agreement, understand that once your 11,000 pounds runs out , your on your own. You are being asked to sacrifice your future earnings for what doesn't seem like a huge amount of money. Look at your options carefully and do what is most advantageous for you. Don't forget that the company doesn't want it's back-room dealings exposed and don't allow yourself to be a victim.