Worcester Highflow 400 problem , advice please?

A two year old Highflow 400 system. Have had no problems until a few days ago.
I use the system manually by turning the heating and hot water to the 'On' position as and when required.

When I turn the Hot Water to 'On', I recieve little or no hot water to the taps but get boiling hot radiators. The Heating is on the 'off' position.

Any ideas what problem is? I will get someone to come out and fix it but dont want to get done over ....being a single woman and all that.

Sound like you have problem with diverter valve

Or the usual one of being blocked solid with muck.