GPS Anti-theft device...

Had a motorbike stolen about eight or nine months ago, only had third party
coverage on it, had just about written it off when I got a phone call from
the police saying they had found it, quite by accident when they were
raiding the guy's property on an unrelated matter - plates had been changed
for an identical model, but there was other info that the police matched it
to me. Officer dealing with the thing said they were very lucky to find it,
the only bikes they generally find very quickly have some sort of tracker,
so decided to look into it

From reading about these devices, and GSM and GPS, I guess the whole gist of
the thing is you send a mobile call to the device and it responds with a
text with the GPS coordinates which enables you to find the location.

Was looking on eBay to see if I could get something I could use, but there
seems to be such a variety of products on offer.

Anyone here got any recommendations for something like this? Ideally
something I can hide that attaches magnetically to the bike and will give me
at least a week between charges, I don't want to have to keep charging it
every couple of days.

The bike was found stored in a garage, so I guess GPS can't get in there,
some facility for sending last recorded position would be better.