Usually when your indicator flashes twice as fast, there is a dead bulb somewhere on the car, switch all your lights on and locate the dead bulb, replace it and that should fix the problem.
Thanks for your reply: I did check all my lights already just before the recent MOT and they were fine. Will check the same again just in case as the problem only started 2 days ago.
Update: spot on: it turned out to be front direction indicator in the headlight ***embly not working indeed. Couldn't believe my eyes as I swapped their bulbs on my own just days ago (the old ones were discoloured). Will have to check the circuitry as its helix still looks good.
I once had an issue with an indicator not working occasionally.
Clean the contacts and then get a small flat screwdriver and gently prise them upwards to ensure a decent connection on the bulb. Over time the springiness goes and they don't press on the bulb as firmly as they should.