Boot sector containing grub overwritten by an MS assistant :(

I 'did' have a dual boot system with win 10 pro and Ubuntu 16.04 (that had been upgraded to I think 18.04 over time). When having problems with win updates I was forced to contact Microsoft support from within win 10 update settings - one of their assistants gave me a call back. That's when the trouble really started! After many hours they decided that the only way to solve my problems with windows was to do a clean install of the latest build which I agreed to reluctantly warning them that I have a dual boot system. They then ignored the dual boot and wiped my boot sector, converted it to NTFS and lost my grub in the process.

I have been attempting to get back into my Ubuntu which was still otherwise intact but was not bootable on its own without the grub. As a last resort I have been attempting to install Ubuntu 19.04 Desktop amd x64 from a fresh iso downloaded from the Ubuntu website this morning. However - thanks to MS and their ignorant blundering through the win 10 install they boot sector is now NTFS and I can't edit the partition at all.

I tried creating a new efi partition for the grub but the install attempt crashed.

Other than scrapping everything, deleting all partitions, wiping windows and Ubuntu totally and starting again what can I do to sort this out. All my data is backed up so a complete install of Ubuntu is not a problem.