Carer's Benefit Extension

Does anyone know if carer's benefit can be extended beyond two years or is the only option to take up carer's allowance?

The difference is that the former is not means tested while the latter is.

The only literature that I've seen mentions 104 weeks - no mention of what happens after that.

Carer's Benefit is payable to a Carer for a period of 2 years only, for each care recipient. This may be claimed as a single continuous period or in separate time periods subject to a minimum of 6 weeks for any given period. After that, Carer's Allowance can be claimed which, as you say, is means tested.