CC & 3DM,
my mate is still in transit, I will catch him next week at work, hopefully he will have more details for us then; over the phone he told me that the owner of the rifle said the calibre was 577-500 No2. Army & Navy and 1890 was mentioned so I presume that was the date that company had the rifle made/sold the rifle; sorry for the sketchy details but mobile phone reception wasn't to great at the time!
Wasn't sure if he even got to look at the action flats for any more info but when it comes to hand I will pass it on to you guys so you can verify data!

Thanks for the help guys, this is why sites like this are invaluable; there are a lot of wonderful old rifles out there sitting in cupboards not being used, now they can be & SAFELY!
Will keep you informed
Many thanks
93x64mm