I said unsupported, but ...

I think I see what the problem is there, you've obviously lost and gone into negative but the vcash column is unsigned. You need to make teh vbookie_cash column on the user table signed. Make it a bigint(20) and you should have plenty of scope for growth ...?
SQL Command:

or do it via PhpMyAdmin

thanks for your help.

where do I find vbookie_cash within the admincp?

not in your admincp - in your database

can i not access the database through the admincp or is it something i have to do through ftp?

I apologise for all the questions - I've never dealt with anything like this before.

right, I've been looking around on here, logged onto my hosts cpanel and put that bit of code into the Phpmyadmin section.

I now have a total of 9223372036854775807.

Where have I gone wrong now?

That sounds right - that number is half of the previous balance as a bit is now use for the sign. Now change your balance to what you want by editing the 'user_cash' figure against your name in the user table of your database. You could manually work out how much people won against you if you want it to be accurate, or just start with 500 again

where is the user table situated so i can edit it?

In your database - use phpmyadmin in cPanel

Removed.

Removed.