Baxi Combi boiler 105he?

I'm no plumber ! Xmas the boiler went dead, no pressure, I turned up the little tap to allow water to flow and built pressure, got a constant trickle feeding thru the boiler, and the leak outside diverted to a drain, and pressure stays spot on, it runs like a dream...is there any downside to this ? How easy is it to fit a new pressure releas vessel on that type of boiler? I got the part of ebay for a fiver, just reluctant to pay £100+ to a plumber ! ! !