Hi. I have had a very slow leak in my central heating for years. Boiler needs topping up 2 or 3 times over winter. doesn't drop in summer. I've found some rust on one of the valves of my towel rail and wondering if it is this causing the pressure drop. I want to isolate this towel rail for a couple of weeks to see if that fixes the pressure drop. However, I cant for the life of me work out how to get this plastic cap off so I can isolate the towel rail (picture below). The other side has a thermostatic valve which I've just turned off. does this white plastic cap just pull off? should I get something to wedge it off? sorry if this is a really simple question. I don't want to break anything and my DIY skills are minimal but I can't get the **** thing off