Have a look at the Givi SatNav bag, there is a review on the forum somewhere. This opens up your choice to just about any satnav you care to use, try going into the like of Halfrauds, Argos, Comet, Curry's etc, they might have something on offer that fits the bill.
If you are looking for IP rated units you have the vast choice of two! (Garmin & TomTom), both of which have followers and haters (personally I think the Zumo is better, but the TomTom has a much better GUI).
There is a pre-loved TT1 on ebay now ->
http://cgi.ebay.co.uk/TomTom-Rider-G...item2ea9261252
or a new unit, but a bit more expensive ->
http://cgi.ebay.co.uk/TomTom-One-XL-...item3a5493fa67
The Givi satnav bag ->
http://cgi.ebay.co.uk/NEW-GIVI-S850-...item53ddf403bd
I suppose, by the time you get the bag and a �100 unit you are prety close to the cost of a TT1......
I guess it depends what you want from your SatNav, in terms of features as well as use.