You'll need to take out your system and pull up the ply,.. no matter how good the sealant is, water will always manage to get under the ply eventually, and when it does it'll be trapped there,... rusting even quicker than if you hadn't done the job at all!
Gardiners stock the small tins,.. might be worth a phone call monday to see if they have the big ones as they're always quick to deliver.