There are some limited circumstances where even though you have signed a compromise agreement you can still bring a claim against your employer. These are generally where the type of claim you have brought is not actually compromised within the greement (rare but possible), where the employer has misrepresented the position and you have a claim for misrepresentation or where the employer does not make the payments under the compromise agreement. such cases are rare.
What is not permissable is to decide 17 years later that it was not such a good deal after all following a change in circumstances of any kind. Here is an EAT case where an employee tried to do just that but had his claim struck out at the Employment Tribunal as an abuse of prcoess. Unsurprisingly, the EAT agreed with the Tribunal and said the claim was correctly struck out.