'Fungus' is never fungus unless it's growing on dead tissue (eg as a secondary infection) - in actual fact you'd have a case of
Flexibacter columnaris and/or
Aeromonas sp.. These are bacterial infections that also cause finrot and the first thing you need to do is very,
very gradualy lower the temperature in your tank to just under 76 deg F. This immediately slows down the progression of the infection. Then dose with an anti-fungus and finrot med. (Like Interpet's Treatment no. 8 - since you live in the UK). Make sure you follow the instructions very carefuly and do frequent water changes even if this means replacing the dose each time.
Melafix has been, apparently, known to cause problems in bettas (which are gouramies) but I, myself, have never had issues and I can't help but think that people have used the product in scape-goat fashion. It only contains 1% active ingredients - 99% is literally useless

And though this isn't a good enough reason to say it's harmless, the actual ingredient (
Melaleuka tree oil - sometimes incorrectly called 'tea tree oil') has only been conclusively shown to be effective against fungus (and I mean fungus - not bacterial infections). You also need to keep in mind that such tests don't generaly look at the sort of pathogens that cause disease in fish - far more money goes into research that'll benefit humans! I use Melafix occasionaly when quarantining fish but I don't bother with it at other times. Bettafix is exactly like Melafix but diluted to 0.2% (5 drops Bettafix per gallon therefore equals 1 drop Melafix per gallon - quite a rip off realy - you can just use one drop Melafix per 5 gallons if you're concerned and forget the Bettafix!).
If your fish is 'coming down with something', you'll probably start noticing more severe symptoms soon enough - I personaly would expect ich/velvet or something along those lines rather than a bacterial infection - but that remains to be seen. You should never treat fish without knowing what they have anyway - especialy where antibiotics are involved.
Having said that, flicking isn't deffinately a disease coming on - it could just be she's irritated by something in the water in which case water changes, immediately, are the best thing. Of course, clean water is always extremely important whether the fish are healthy or not.