Doh. How stupid do I feel now you point out the blindingly obvious! Of course we just want the low order bits for the wrap as you point out. To make it even worse there is already and seed & 0xffffffff immdediately above and this was my first thought of how to do it. I rejected it due to temporary insanity.

Many thanks. The sawtooth pattern was pretty, and predictable given what I was doing.

Cheers

James