Simple"v=spf1 +a +mx -all"
When bytehouse antispam used
"v=spf1 +a +mx a:mailers.bytehouse.co.uk -all"
When bytehouse antispam & bytehouse exchange used
"v=spf1 +a +mx a:mailers.bytehouse.co.uk include:spf.serverdata.net -all"
When bytehouse antispam & bytehouse merak used
"v=spf1 +a +mx a:mailers.bytehouse.co.uk ip4:93.91.23.159 ip4:188.191.154.26 -all"
Custom SPF record matching our requirements could be manually created using the tool http://spfwizard.com