Hi Clare, and welcome to the forum.
The Mk1 rifle is one of my favourite rifles. Manufactured between the mid 1920s and early 30s, they were Webley's first foray into air rifles. Diminutive and not particularly powerful, they are a pleasure to shoot. They are one of the rarer Webley rifles and reasonably sought after so command a decent price. In my opinion, one of the best things you can do with one of these is use it now and again. Mine comes out at Bell target now and again and it manages to pull off a reasonable score given it's shortcomings.
Fantastic that it's been in your family for so long, and hopefully will be appreciated by generations to come.
Dave