I am a Chiropodist. And I don't have the silver bullet answer. After speaking to many patients over the years, it seems that people have had random successes or failures with the various methods and treatments out there.
And yes, all of the above suggestions in this thread probably have had good outcomes in some cases, but I don't think any method is a sure-fire thing... sometimes they work for some people, but others don't get the same result.
I had not heard of Milton Sterilising Fluid as a treatment for fungal nails. I'm aware that it is a disinfectant rather than being antifungal. But if it has worked well for some people, I can't argue with that.
There are quite a few variables - perhaps the fungus has gone deep into the nail root, or the nail thickness prevents enough penetration of topical treatments.
Another variable is that there are different strains of fungus. So different active ingredients in anti-fungal creams, sprays etc. may be effective for certain strains, but not great with others. So if you've tried a certain cream or spray for months with no result, then try a different one that has a different active ingredient. A friend or someone may say, hey this worked for me, but it doesn't mean it would necessarily work for anyone.
Oral Terbinafine probably has the best successful outcome on average. However, I wouldn't use it myself because of the slight risk of liver problems. I myself have fungal toenails, and even the low risk of side effects from using Oral Terbinafine does not seem worth it.
As mentioned, nothing is certain to work effectively. I have a go at my nails often (it's starting to work but verrrry slowly indeed). I've used Daktarin spray, Mycota cream (from Home Bargains) and Tea Tree Oil. I'm consistent in the warm weather, but tend to slacken off when it gets cold. That's me not being compliant with my own intentions.
If you do try Tea Tree Oil, probably best not to use a cheapo type - the cheap bottles have 20 to 40% actual Tea Tree Oil. It's best to try 100% neat oil... costs more of course. And if the nails are pretty thick, maybe have them burred away to make them thinner. A Chiropodist / Podiatrist can do this for you, or you can buy little drill gadgets - go very cautiously if you buy a gadget, as the abrasion can be damaging or get very hot and burn the skin under and around the nail.