Hi Julian,

Kayaking in Scotland is mostly rain dependent (except for a few regular dam release rivers - which go in the summer).  This means it is unpredicatable.  Having said that, the times of year when rain is likely are Autumn through to the Spring, with Oct / Nov and Mar / Apr often being pretty wet. 

For example, the last week has been cracking for paddling with lots of rain and some snow melt.  The best guide to rivers is the Scottish Whitewater Guidebook (2nd Ed) which you can buy from here: http://canoescotland.com/Default.aspx?CatalogItemID=26&CatalogID=3&psnavcmd=CatalogItemDetails&tabid=379.

The ideal thing is if you could watch the long range weather forecast and hop on a plane at short notice.  If you have to plan in advance Nov / Dec is a reasonable bet for water.  Once you're here a combination of the wheresthewater River Levels page and the local forecast should help you get the best out of whatever rivers are going.

It depends where you are going to be based in Scotland but there are lots of paddlers about and you should be able to hook up with someone.