flushing the toilet makes the hot water tap flow on its own!!

Sound as though the Problem is with the tap (probably compounded by poor water pressure or bad plumbing design), when you flush the loo the pressure drops (change from standing pressure to dynamic pressure)in the cold water pipe whilst the cistern re fills. I'm assuming you have a combi boiler? in which case the pressure will also drop in your hot pipes. I would hazard a guess that your tap washer is relying on water pressure to hold it against its seating, hence when the pressure drops it lets by ! Hope this helps

It could be lose of water pressure from when the toilet handle is pulled, the pressure is less in the pipe as its taken to refill your cistern. The washer in the tap will need changing, not a big job. Regards Ian