"If only Grandad was still alive and could do a DNA test himself…"
If either of your parents are still alive then that will likely make a huge difference. I really would recommend that you ask your parents if they would be prepared to do a DNA test as well.
That could also really help with identifying further relatives of your great grandfather.
A few years ago my parents both did a DNA test with Ancestry (they were born in the late 1930s, my sibling and I were born in the mid 1960s). Between them, they have so many more DNA matches than I do.
For example, on Ancestry I have 22,791 matches. But, my dad has 21,896 matches and my mum has 25,870 matches.
Just by going back a generation I have gone from having 22,791 matches myself to 21,896 + 25,870 = 47,776 matches from both of my parents.
(Although, technically I have 23 matches from both sides so only 47,753 distinct matches).
For me, going back a generation with the DNA test has more than doubled the number of matches.
"I can’t find a death certificate or anything else about him ... I recently found the electoral registers and have been able to narrow his date of death to a couple of years, but still no obvious death/birth certs."
Have you tried looking on the GRO website?
You may need to make a free account to search, but if you go here:
https://www.gro.gov.uk/gro/content/certificates/indexes_search.asp
You can search for both birth and death certificates free of charge. With birth certificates you can also search for the mother's maiden name and with death certificates it will give you the age at death.
"Sorry, bit of a rambling message but I needed to tell someone…"
Don't worry at all, I totally understand that feeling and have also used this forum to vent in the past.