Starting to sound like it's flared then.. I'd try a FSA Pig headset (the one with sealed bearings ideally) That should sort your problem, unless it's flared really badly.
Flared frame (not deffinitly, but if there's been a headset that's ever so slightly bigger than the king, it will have pushed the headtube out to a slightly bigger size, if that makes sense). Get a FSA Pig headset, it'll be fine then
Boiling water will get a lot of impurities out of the water... And, It may well expand when it gets hot. But how hot does it get in the uk.... Not hot enough.
Because the pistons move out. Put a hole in that lid and put a tube or straw etc in the whole into the bottle. Now freeze it... Work it out, scientist.
I've known people to get sticky pistons with oil bled maggies. I'd say its more dirt etc on the pistons rather than not being lubricated. What happens when you split a hose with an oil bled maggie.... Oily mess over rims/disc... Hellooooo contamination.
I kill my rims by flatspotting them and bending them before I destroy the sidewalls.. Especially with echo 07 rims, will take like 20 grinds and a grind will last over a month... That's twenty months. If you ground it every 2 months, then its 40 months.... about 3 and half years.
Water gives a better feel to the brake, returns quicker, easier to pull etc. It's also free and if you don't have the syringes etc, you can't exactly fill your sink with oil.......