The company insisted on signing a 5 year bond for approximately £12000 none reducing.
Good for them, but it only became a contract when you added your signature, thereby agreeing to the terms contained within.
I have worked for this company for nine months now and am considering a move to another company for a better salary.
That happens, and sounds quite reasonable and understandable.
I know the company would try to enforce this bond.
Not if you pay them what you contractually agreed to. There would be nothing to enforce. They or you might grumble, but since you fulfilled the terms of the contract, that would be the end of it.
What does everyone else think?
Like them or loathe them, you knew the terms you were entering into when you signed up. If the terms were bad, then you will presumably learn from the experience.
If it were me I would work out how long it will take me to write off the contract break sum based on my new improved salary and move on. Aviation is a small world, and reputations are important. They can worry about theirs, but I want a good reference. I want to avoid the stress, cost and uncertainty of protracted legal wrangles. I want to look forward to an improvement in my life.
That is what I would do, but I can't tell other people what they should do.