Hi,
I looked into this a little further - I check the server and it turned out the php script memory had not increased to 100M but was in fact still at 16M - To allow the software to run I have now set the server globally @ 32M and set the site map generator folder (only) to 100M - I have just managed to successfully create a 12,500 url sitemap without a problem and later I will attempt to let the software generate an unrestricted sitemap...(I expect around 30,000+ pages)
I'll report back on progress.....but so far so good!