Digging around, it seems that your domain's owner is Cuepower.com, whose phone number is +44(0)8448 003 933. Shoddy practise on their part to register themselves as the owners instead of the school. However, they should be able to give you the login to update the site.
Here are the full details:
Domain name:
wrenspinney.co.uk
Registrant:
Cuepower.com
Registrant type:
Unknown
Registrant's address:
Edit: Address Removed
Registrar:
Namesco Limited t/a Namesco Ltd [Tag = NAMESCO]
URL: Namesco - Domain Names, Hosting, Dedicated Server Solutions and Broadband ADSL, Namesco Limited
Relevant dates:
Registered on: 12-Sep-2007
Renewal date: 12-Sep-2011
Last updated: 03-Sep-2009
Registration status:
Registered until renewal date.
Name servers:
ns0.phase8.net
ns1.phase8.net
ns2.phase8.net