After a lot of searching I found out this number should be 0870 900
0
037.
Saga's relevant website is
here.
At the bottom of this page it states that the insurance is underwritten by London and Edinburgh Insurance Company Limited.
That doesn't mean that they take the calls if Saga don't though.
I'm going to continue searching a little more.
Update:Saga's contact us page
here mostly lists freephone and 0845 numbers.
0870 isn't listed though but a
google search for 0870 900 0037 does list it in a press release.