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So you are actually researching the Fisher line.  If Jack (or was he born as John?) & Mary are now deceased what exactly are you looking for?

If you are looking for living descendants - we can’t help as Rootschat has a “no living persons” policy.

You can use freebmd to look for marriages for them and to look for children born to those marriages

As your gran is still alive - is she not able to give you that info?

Your gran will know her parents names and mothers maiden name so you can use freebmd to find their marriage and then trace her ancestry via the census info which can be found on wwwfamilysearch.org

The Arnold/Fisher marriage was 1946.  What year was Jean born and was she born in Lincolnshire?

Your grandfather was born 1920 Grimsby

What year was John J Fisher born and who did he marry?

You can look up your grans birth reg on freebmd which will give her mothers maiden name

www.freebmd.org.uk

Edit

Just realised you have used present tense for your gran so endorse Rosie’s comments below