Hi Phil,
I could have predicted that. The sealant used should be able tolerate a reasonable amount of shrinkage. Is there any evidence of shrinkage elsewhere in the bathroom, is the bath secure i.e. does it wobble if you stand in it? Reasonable is generally considered to be the width of a 50p coin.
You could pursue this through a small claims court -
Making a Small Claim | Consumer Information
You'll be staking the cost of making a claim though.
Tony