eviction sub letting

Rent Arrears

Hi have a tenant who has made every excuse especially over the last 18mths not to pay rent. Finally got housing payment via DWP for a proportion. Electric is also managed by myself and this is also mounting up unpaid. Total outstanding now is £4500. Just found out after goin g to the property after no reply to texts phone calls that somebody else living there - and paying the tenant money to live there. However still no money coming my way. What Id like to know is
Is there an immediate eviction for sub letting
Can I change the locks on the property as this person has not been checked and not who I let the property out to
Original tenant still no reply to messages left either text messages or voice mail
If I need to evict giving notice does a registered letter send via post qualify for evidence of eviction as he does not reply to emails , texts , phone calls and not there at the property to serve in person.
Nightmare !