Hi Paddy.
I guess that you'd better work out whether you want a sail for stronger or lighter winds before you go any further. Whichever way you go, I would recomend staying with sail of the same model, or one with a similar feel to your Tushie 5.5. (ie, a Rock)
As importers for Tushingham, (along with the many satisfied owners) I know that their performance, ease of use, durability & wind range are the qualities that always places them at, or very near to, the top of the reviews.
http://www.tushinghamsails.com/reviews.asp?id=20 Like other posters, I would also say that a 4.7 would be the smallest that you should go for as you will get a lot more use out of it,
(unless you're a fly-weight).
I would recomend either the 4.75 Storm or the 4.7 Rock. Both have a phenominal wind range, with the Rock having a slightly smaller boom length.
www.tushinghamsails.com/reviews.asp?id=23There are other variables as well. As Nebbian say's, down haul, along with the correct mast, should give the Storm a pretty wide wind range.
Another important factor, especially for (weedy) Safety Bay, is the fin.... there are a lot pretty mediocre weed fins out there that do nothing to enhance a boards performance, especially in the stronger wind conditions.
Feel free to come in & demo the sails.
Regards.
Jez