If you are a gold buyer and took this in for £500 from some poor sap then I think you are quids in. Assuming you checked it actually is 18K. If you are private citizen without the equipment and wherewithal to check then I worry for you. As above, post some pics of the back and ideally the inner caseback. Since you know the year, you must have already removed that. It might still be a win since I suspect there is at least 10-15g of gold there.
Note the bracelet fitted isn't original, at some point that was on an expander like a Speidel hence the damage to the lugs. One thing that worries me is that people with solid 18K watch heads don't often put them on nasty plated bracelets...