Can the Contractor issue a CE instructing the Subcontractor to work nights - to reduce the programme? There are no references to night working in our Subcontract or programme.
Unless it is illegal or impossible very difficult to refuse an instruction. You just have to price accordingly, including demonstrating/pricing the additional risk/disruption/cost that you will incur. This will clearly be a compensation event as is a change to Works Information (new constraint) but obviously your concern will be that they can assess a quotation themselves if too high/too much risk and then only adjudication can overturn this.
If you see night working as being problematic you could notify an early warning to discuss the potential concerns in order to be able to table and discuss the matter and agree some actions as a result.
To add to Glenn’s answer, in your quotation, as well as a allowances for risk under clause 63.6, the assessment is also based on the assumptions in clause 63.7 amongst which is that the Accepted Programme can be changed.
So if you just do not have workers available to work nights, then the AP cannot be changed.