Lettings agency/landlord asking for £250 fee

[Jenesaisquoiii]

I thought changes to a tenancy were capped at £50.
The agency says it's going to be £250 to add a sharer/occupant onto the tenancy. Was previously 1 person alone on the tenancy.
Has anyone been in a similar situation in the last few years?
Did they have a reasonable cost that they had a receipt for? I can't understand why it is five times more.
Many thanks in advance

You are correct, fees are capped at £50 unless the landlord/agent can show the change costs them more than this. Are they having to undertake any referencing or additional checks for the new tenant? Still, £250 is a lot to just do that.