Local Housing Allowance

Other

i have a 2-bed house which i rent to a long-standing tenant under an AST for £70pw (paid by the council as housing benefit / local housing allowance, call it what you will), which i know is cheap. Recently, my tenant has fallen ill & now has had his son move in with him as his carer.His son receives a carers allowance etc & the housing benefit dept are aware that he now lives there.
I asked the council recently what the Local Housing Allowance would now be on this house based on the new circumstances & this is the reply I received below:

The Local Housing Allowance Rate applicable to this claim is £138 per week. We cannot pay more that £15 a week more than the rent charged and your tenants weekly entitlement to Housing Benefit from 13/4/09 is £85 per week (rent £70 plus £15) From 13/4/09 onwards you will be paid £70 per week and your tenant £15 per week.

For the period 30/3/09 - 13/4/09 your tenants weekly entitlement to Housing Benefit was £87 per week - the payment was split between yourself and your tenant - your were paid £70 per week and your tenant £17 per week.

Can anyone tell me exactly what this means? Does it mean that I can seek a rent increase to £138 per week? If so, how do I do it?