ok, i may be getting somewhere now......or possibly not!
I have since found out that one of our xml imports fails everynight due to it trying to import the file whilst its still downloading and therefore not running the sp_xml_removedocument.
I've been monitoring the VAS every minute and when this failed on tuesday it had zapped the largest available block and not released it. So, i monitored it last night and nothing, no reduction in the total or the MAX available........and it actually failed twice last night
Could this just be coincidence? I'm going to continue to monitor it over the next few days and we'll put some error handling in place to fix. I don't want to put the fix in just yet as i want to see if it breaks everything again.