For the A record for legsreunited.org.uk ,do I press Remove and then go to Add new Record and use the IP of 16.170.197.84 ?
Yes.
But remember that you have to add it to your list of domains at jalbum.net. You have only added www.legsreunited.org.uk now.
https should start working within maybe 10 minutes after have the "green mark" that the IP is correct.
If so what is the TTL and should that be changed and what to?
The TTL (TimeToLive) value doesn't matter for the functionality. It is the number of seconds the IP should be cached before a DNS server should ask its parent server for the value again. Short time is 1 minute (60). Long time is 1 day (86400). A short value can be good while testing, so if you have to change the IP you don't have to wait too long for the value to change. When it works you can raise the value so the value is cached longer.
Presumably, I will then have to do the same for my other 2 domains?
Yes...
But start with one and test it so you know if and how it works.