Linux sysadmin

We look after a number of Linux servers at customer offices and regularly need expert help, we can do the basics ourselves. Work load varies from week to week, from nothing to 20 hours. This project is for 10 hours of remote sysadmin work, if this works out there will be ongoing work. You need to be "on call" available as required, 24 hours at the latest, ideally at 4 hours notice, depending on the urgency of the problem. Our time zone is London, UK GMT +1 You need to be a Linux expert/guru. Our servers run all the usual good stuff: iptables, Sendmail or Postfix, Squirrelmail, Vacation, SpamAssassin, Clam AV, F-Prot AV, Squid, Samba, Tape backup, Raid monitoring, UPS monitoring, etc, plus some stuff I've probably forgotten.