Can I claim mileage?

Gb1623 wrote: »

Sorry I invoice the agency who invoice the practice who pays them directly

That is not quite the same as you asserted previously in this thread

Gb1623 wrote: »

I am a sole trader. I don't have a pattern to the practices I go to I travel anything from 2 miles each way to 40+ miles each way. I invoice each practice and they pay me directly.

Thank you. I appreciate all your help

Anyway, moving forward.

Are you required to attend the agency each day to receive your instructions for that day?
Or do you travel direct from home to your place of work (i.e. the dental practise you are assigned to for that day) each day?

Gb1623 wrote: »

I travel direct from my home to the practice.

Then I'm sorry but as a person who is working through an agency, "the first trip of every day, from home to work, and the last trip of every day, from work to home, is considered a commute so cannot be a travel expense."
Source: http://www.contractoruk.com/expenses/agency_workers_travel_expenses.html

But I'm not sure this situation has changed recently

What did change effective April 2016 is that the government cracked down on dodgy agencies who were abusing the tax position. It affected those employed via an agency umbrella (and indeed those using their own limited co but were caught under IR35)

Essentially it clarified that if you are a temporary worker subject to SDC (Supervision, Direction or Control) then you cannot claim travel or subsistence tax relief.
More info here:
http://www.nixonwilliams.com/free-resources/2016-tax-changes/april-2016-changes-travel-and-subsistence-expenses

That's because those subject to SDC are probably, imho, disguised employees not self employed as you assert you are, and it should be the employer that pays such expenses.
(Commuting is never an expense in relation to tax affairs)

If you are working through an umbrella company, then that umbrella company is your employer. i.e. you are not self employed.

Why not get your neighbour to employ you ?

Each day you call in ( next door ) to your place of work, and are then sent by car to the assigned job.
When have finished, you call back at your normal place of work, then "commute" home.

I can't see why that won't work, but I'm sure there is a law somewhere.

You can't claim to get to and from your place of work no matter how many places there is. If when you get there you were sent to different places such as people homes you could claim for those journeys, but as you work from the practice you get to then this is just a normal commute.