I will try to answer this question although I am unsure if the agreement to reduce terminal float occurred before the subsequent CE’s. For the purposes of the answer I will assume that it was. It should also be pointed out that this has been done by Contractor agreement – not something they had to do or indeed are obliged to do again.
The key milestone on the programme is “planned Completion”. No compensation event ever on its own moves Completion Date. The compensation event is assessed as the movement on planned Completion, and if it has moved then Completion Date moves by the same amount. This is very clear in clause 63.3 (or at least clear once you have read it three times and are familiar with it).
If you are saying that this is the first time that you want them to reduce terminal float – i.e. not move Completion Date by the amount that planned Completion has moved then again this has to be by agreement – otherwise you have to do what the contract says and if it were to go all the way to adjudication then that is what the adjudicator would look at.
The other factor to consider is what benefit does this actually give the Contractor? In the assessment of CE’s they would get the full value of any cost incurred which will include prelim type costs and fee. The only benefit that terminal float gives the Contractor is the protection from any delay damages as highlighted in X7 for that project for a longer period.
So to answer your final question as to how this can be taken forward – it would be by agreement if you are looking to do something different to what the contract says, and if you can’t agree then the answer is to do what the contract says otherwise you may end up in adjudication.