Sorted now pretty much. Tried GoDaddy.com and although the setup is far easier than StartSSL its the same so actually I've wasted about Â£4.50 but I'm not bothered as it won't need renewing for 12 months, apparently StartSSL is a monthly renewal process?? Not sure but that's the general vibe I got through searching although information is so patchy it's hard to pick out the relevant bits. However at Â£4.50 I'm not complaining!
Only thing now is the re-direct on HTTP to HTTPS doesn't seem to be playing ball. I get an error message asking me to use HTTPS before the address, which when I do logs in fine. There's no automatic redirect which can be annoying but the site is bookmarked and it's only me so no great shakes. It might be down to an option on the NAS somewhere but everything obvious is selected.
The SFTP warning is down to software I'm using specifically ftpcafe on Android and FilezillaFTP but a manual check and saving the details allows uninterrupted log in the next time around. All secure.
I know a few people have Synology devices on the forum so hopefully anyone thinking of doing the same will see it can be done and I post further details if needed. More importantly all done for free