Hi Redfan.
I've re-read your first post an honestly say I'm a bit concerned that your vet didn't immediately treat for worms - all the symptoms you describe could well be down to the infestation - especially as he did find fleas and all vets know that tapeworm is transmitted by fleas. The two almost always co-exist. Cats with severe worm infections do become tired, lethargic, diarrhea ridden, and because of their low health can be prone to infection so could display a higher than normal temperature too.
Don't want to be presumptive but I had a vet once who made a habit of stringing out treatments for his own ends, while my cats continued to suffer pain...I do hope that's not the case here.
Depends if you can effectively "pill" your cats or not - I suspect this isn't something you've tried before. It's not the easiest skill to master and some cats just will not be pilled, so be ready for this.
If you CAN give them pills, this is surely the most effective way because pills such as Drontal contain ingredients that kill all known parasitic worms in cats. You dose by weight, so make sure you buy the correct mg pills or you may risk under/overdosing the boys.
If you can't get to grips with pilling, and believe me we've all been there and failed at that, I use Droncit spot-on which you apply in the exact same way as Frontline - just behind the neck on bare skin. Droncit spot-on ONLY kills tapeworm, but as this is the most common worm to trouble for in/out cats (due to it's eggs being transmitted by orally ingested fleas) it is a pretty good second best.
I tend to use Droncit spot-on three times a year (larger cats need two tubes), and when my cats go for their yearly vaccs/check-up, I ask the vet to pill them a multi-wormer (£7) so they get a proper clean out annually.
Droncit spot-on, Drontal tablets and your regular flea treatments (Frontline Combo, Advantage, Fiprospot are all recommended) are available without prescription from Animed or VetUK. I find Animed to be cheapest and very prompt in delivery.
http://www.animeddirect.co.uk/cats/wormingIf you think the fleas are now in your home environment, order a can of Indorex while you're at it. You will almost certainly need it when the warm weather finally arrives. One application will clear their beds and play areas of fleas, eggs and larvae for a whole year.
Don't be afraid to ask questions, even what you might think are daft ones - it's the cats that matter and we are all learning every single day, and all find the need to ask "daft" questions from time to time.