It would appear you need a 6inch fan as 4 inch only do up to 75m3/hr,screwfix do a 6 inch xpelair which should suit http://www.screwfix.com/prods/55160/Heating-Cooling/Extractor-Fans/Xpelair-SL150-Standard-Slimline-Fan-Remote-150mm;jsessionid=JIBAAYKVODASECSTHZPCFFQ?ts=67306
You have to open up your O/S light,if there are only 2 cores(+earth)and they go to common and L1 or 2 in your lightswitch you do not have a neutral and it is wired wrong.check your O/S light + let us know.
If u stay in Glasgow there is a company in Southcroft Rd Rutherglen called BEMCO who stock spares for these if u tell them make and model they may have a fusible or thermal link and when you see it u will be able to find it in the heater and change it after making sure power is off.Short of...
NO as far as I know you should not spur off a cooker cicuit esp with 2.5mm cable although I know it happens.Could you not spur off a ring circuit for your cooker hood spur
Just to add that the permanent live could also be in your outside light you would also need some sort of voltage tester to check this.If you do find a permanent live it should be connected to the red of the cable going to common of light switch and the link removed,the returning black...
Another thing I would do here is put the alarm on a different breaker it should really have it's own 6A breaker if none spare put it in a 6A lighting circuit.Your battery is def the problem with your alarm going off.
This sounds like a short circuit probably L-N inside your wall,possibly a nail through a cable but it is surprising that no breakers trip(I take it you would have mentioned if they did).To find it you would have to narrow down as far as poss where the bang is coming from, isolate power then...
I,m going to start from the begining and explain this as simply as I can.Does your existing 2G switch have Permanent L+N feed(Your permanent live will be the red wire in the terminal marked C(Common) on the switch which has only 2 wires going to it and probably operates your downstair hall...