You should be eligible for compensation for the first failed install (Nov '23 to Mar '24). You should also be eligible for the second install if this too is in delay. In answer to your questions:
1 The number for installations is the pre-installation and delivery team on 0800 052 1734. However, those taking calls seem unable to change/influence anything about your installation. They simply relay the next date that pops up on their screen. This can often be an automated rolling date showing the day you make the call and the call handler will 'guarantee' someone will attend but, time and again on here, there is no reliability to this and usually no one turns up. Generally customers can't make contact direct with the installation sub-contractor. In some past topics on here the customer has made progress via the sub-contractor but this normally seems to happen by random luck when the customer happens to bump into the 'right' person from the sub-contractor on site to make things happen.
2 Topics on here regularly describe VM trying to dodge paying out the required compensation. It is normally the case though (from past customer feedback on here) that when the case is heard by the ombudsman the customer gets a fair hearing and the compo is awarded. Key to success with the ombudsman seems to be keeping an accurate and detailed record of events (with evidence) so the adjudicator can easily follow the sequence of events that has taken place. The compo is due even if the customer or VM cancels. I believe it is possible to request payment by cheque rather than credit. If you are due compo but never actually become a customer then you are paid by cheque.
3 VM has been taken to task in the past by the ICO for poor handling of DSAR. Topics on here regularly mention difficulties getting any/all of the information requested in a timely fashion. Usual advice would be to record any calls to VM yourself if you can.
