Hi guys. I'm hoping to get some advise on what might be causing my Bora V5 to overheat after literally only 5 miles of driving. I'm not particularly mechanically minded (which may be partly to blame for it getting to this stage) but I'll give you a breakdown of what's happened so far:
1. The low coolant warning had come on maybe a couple of times like six months apart. I had just topped the coolant back up to max, thinking it was only minor.
2. However last week, I'd noticed that there was some 'gurgles & burps' coming from under the bonnet. I had planned to investigate further but...
3. At the weekend, the car suddenly overheated (dash temp meter at the max), despite there seeming to be plenty of coolant in the system. It maybe worth noting that the AirCon was off prior to this as I'm not a massive user of it. Roadside Recovery came out and diagnosed (via pressure test) a leaky radiator which I've since had replaced today. The mechanic who replaced, left her idling (with AC on this time) for 20mins or so after the job. No obvious issues.
4. I took her out for a test drive tonight to make sure all was well, and unfortunately it overheats again after only 5 miles of driving (AC off again). Again, there's plenty of coolant in the tank. I check underneath for leaks, can't see anything obvious, but there is a bit of a 'gulpy/hissy' noise from behind the engine block. I trace this noise to the pipe leading back to the coolant tank.
5. Leaving her to cool down for a bit first, I make it back home without her going over 90degrees on the dash, thanks to being able to coast the majority of the route. However, after a bit of research, I'm still a bit confused, but am now thinking it could be one (or multiple) of several parts

a) faulty fans - I've now seen that there is a common issue with the slow speed on the fan unit on a lot of Mk4s. Doing a brief observational test on these after getting home this evening (car was still warm & just idling), I've noticed that they do only seem to come on for brief periods when I have the AirCon on (I don't use it very often) or when the dash temp starts to get towards the max. It's not on constantly... Not sure if this is more of an AirCon issue than the overheating problem though because...

b) faulty water pump - ... I have read a couple of things that said that if the car is overheating that quickly with plenty of coolant in the system, then it probably isn't the fans and is more likely to be the water pump... or instead, it could be the...

c) thermostat (stuck shut). I had read that it was a 'wax' thermostat. Not exactly sure what that means or how it works, but it being stuck shut doesn't sound good.

Other parts I've seen mentioned that others have replaced with varying degrees of luck:

Temp Sensor - is this what feeds the dash meter reading? How does this impact on the rest of the system?

Thermoswitch and/or Fan switch - unsure if these are the same thing or not? Is it part of the fan unit, and so replacing that part would replace this switch too? Or is it part of the radiator?

Sorry, for the overly long 'cry for help' type first post. I just really want my V5 back
Grateful for any advise
Rob