Hey Pete,
I would start by bleeding the front brakes right through, and using some Dot 5.1 fluid (halfords sell it) it has a higher boiling point than standard Dot4, and it could just be that the fluid has leached water over time, as brake fluid is hydroscopic.
If that doesn't help, the next cheapest solution is chaning the pads for some EBC HH sintered, these did more for my old 97 Blade than anything else i tried at the time.
Also the lines could be getting past it, by now, might be worth putting race setup on (one line per caliper) cheap enough on fleebay.
If all else fails to stop the bike, look at the master cylinder seals, if they are ok, then the only fix is replacing the stock master cylinder with a (19x20 bore) Brembo master cylinder, but we're talking £200 a pop + fitting, but that will stop you on a sixpence.
Hope this was helpfull