Rented flat, agent or some other let themselves in.

[Room101isWhereIUsedToLive]

Just got home. They needed to do an electrical inspection. Originally they werr meant to be coming last friday, I misunderstood and arranged for time off this friday. But spoke to letting agents last saturday and they told me landlord ( who is apparently a qualified electrician) would not be back from holiday until the fourteenth. So would arrange a date and time after that.
Jyst got home, there is a pen I have never seen before and my toaster and kettle are unplugged.
So they have been in but does the initial misunderstanding over date and time, mean that legally they could enter?

Hopeful bump.

No. Only in an emergency does the landlord or his agent (that means anyone working for the landlord) have the right to enter without your permission. Sounds as though there has been a real mix up over the date as the electrician attended on the date you had arranged time off so the managing agents haven't communicated things clearly.

Contact the agent and express your displeasure. Make it clear you do not give permission for anyone to enter unless you are there to let them in. At this point they'll probably start saying you've agreed to it in the tenancy agreement, but by law you are entitled to quiet enjoyment and can refuse.

Change the lock barrels on the door so this can't happen again, but keep them so you can put the originals back when you move out in the future.