Any reason why you can't tax a car on a temp insurance?  So for example if I bought a car at auction with no tax, could I tax it on a days insurance cover note to get it home legally?

No mention of temporary cover here;

These are usually a printed or laser-printed document, or a plastic credit card style document.

Insurance certificates downloaded directly from the internet by the policy holder are acceptable from 30 April 2010 (this applies to GB only). Fax copies are also acceptable.

The following documents aren't acceptable:

•policy schedule
•renewal notice
•receipt for payment
•amended certificate or cover note - unless stamped or initialled by the insurer
•‘legal protection’ policy certificates
•foreign certificates
•photocopies
•Green Card (International Motor Insurance Card)
Lost, stolen or damaged insurance certificate

Get a duplicate insurance certificate or cover note from your insurance company.

Waiting for your insurance certificate to arrive in the post or update online

You can't tax your vehicle until your insurance certificate arrives or your insurance cover for your vehicle has been updated on the Motor Insurance Database. You must keep your vehicle off the road in the meantime.

If you are taxing your vehicle using the DVLA’s vehicle online service, then you’ll need to wait until MID has updated before applying. You can check online to see if your insurance has updated.