I'm a patient at Forest Hill Rd and have always been able to get an appt for the same day when I phone them early and tell them it's my 4 year old who is poorly. Or the duty doctor always calls me back in 1 hour, telling me what to do. I'm happy with them.
We took the potty everywhere! And got her a book and nice stickers. It was done in the summer, though. Good luck, it seems messy but it's doable, so don't despair!
Lots of fluids (water, diluted juice)and steam (i.e. from shower or warm mist humidifier) to ease the mucus. It worked when my daughter was around this age.
In my experience, sometimes the health visitor is not right... Also, I've heard from a dentist (in my home country) that the discolouration occurs when kids get their permanent teeth(i.e. not the milk teeth).
When we bought an oil filled radiator there was an instruction to turn it on its highest temperature for X number of hours (can't remember) before starting using it. It smelled really oily, stayed away from it, but it worked. Now it doesn't smell anymore.
It's happened here too... I tried the next size up and also a different nappy, Pampers Day and Night (or something like that). It worked for us. Good luck!