Thanks for your patience. Initially you can argue that they have acted in breach of contract by not paying you what you were due contractually. However, my main concern is that it was 5 years ago you should have started receiving these payments and nothing has happened since. They could argue that legally you have affirmed their breach, meaning you have practically accepted it and the terms of the contract have now changed over time and no longer reflect what was agreed 5 years ago. Your position would have been much stronger had you challenged them as son as it was clear they were breaching the contract and taken action soon thereafter. Having waited so long with doing nothing about it means that your rights may have significantly weakened in relation to challenging them over these payments.
Still you could use the newly proposed changes to consider resigning if necessary. That is because pushing changes through without your consent or the contractual right to be able to do so, can amount to a breach of contract too.
It is worth mentioning that there is a possible alternative solution, where the employer is approached on a 'without prejudice' basis (i.e. off the record) to try and discuss the possibility of leaving under a settlement agreement. Under such an agreement, the employee gets compensated for leaving the company with no fuss and in return promises not to make any claims against the employer in the future. It is essentially a clean break, where both parties move on without the need for going to tribunal. However, it is an entirely voluntary process and the employer does not have to participate in such negotiations or agree to anything. It just means that these discussions cannot be brought up in any subsequent tribunal claim and prejudice either party. So there is nothing to lose by raising this possibility with them as the worst outcome is they say no, whereas if successful it can mean being allowed to leave in accordance with any pre-agreed terms, such as with compensation and an agreed reference.
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