Britax Xtensafix child car seat withdrawn from sale after independent crash tests

The Britax Xtensafix child car seat has been pulled from sale after it failed a series of Which? safety tests. The £216 seat performed so dismally in one Consumers' Association test that Which? had to slap it down as a Don't Buy.

Britax is offering a refund on all Xtensafix seats. Why did it perform so badly?