fuel tank leak

If I fill the fuel tank on my BX1800 the fuel leaks very slowly out around the steel disc with wiring attached, under the left fender. I haven't yet taken it apart but it looks like a white plastic washer and a black rubber gasket that would seal the connection.
I have just been filling the tank about half way and it doesn't leak at that level.
Anyone else experience this and know what the solution would be?

Ed- thats a big help thanks.... I assume you are talking about the serial number? Mine is 5A257, how does that correlate to the 5 digit number you listed?

I picked up the kit mentioned on yesterday at the dealer, got home last night and slid the tank out- it already had the replecement kit in it! I replaced it anyhow and used permatex gasket sealent, let it sit overnight and put fuel in it this morning. I think it still leaks though- I mowed all day with it and the fuel gauge still says over half full so I dont think that's working right either.
It's a bit of a project to get at the fuel tank so I dont look forward to doing it again...... looks like I will be though. I will probably refuel it first and double check to see if it is leaking (and where).
BTW, the kit I removed had one rubber o-ring on each of the five (or six) little bolts whereas the new one had two rubber o-rings on each.

Well I filled the tank today I still have a leak at the fuel gauge assembly. I took it apart, cleaned it up and tried it again and it still leaks. I watched it carefully as I filled the tank and it looks to me like it leaks between the steel plate and the white plastic disc.
Anyone have any ideas what I might try to fix this?