How Do I Purge and Re-Light a Worcester 28CDI?

I returned home from work today to discover a note through the door stating that as they had a report of gas they turned off my gas supply (I have the meter outside) and the system needs to be purge and relit.

I've tried running a bath but no hot water - the boiler does make a clicking noise as if it's trying to light but doesn't.

Any ideas how I go about urging and relighting the system in order to have hot water / heating?

Many thanks