490 has a good example for contractors (page 15)...
Erica is employed as a computer consultant. She works full-time at a site for 18 months developing a new computer system. The work is then extended for another 18 months at the same workplace, for the roll-out of the new computer system.
The roll-out is subject to a separate contract between the employer and client.
As long as Erica did not expect to be working on the site for more than
24 months she is entitled to relief for the cost of travelling from home to the site.
Once her employer enters into a new contract Erica expects to be working on the site for more than 24 months so from that point she is not entitled to relief for her journey from home to the site.
The roll-out is subject to a separate contract between the employer and client.
As long as Erica did not expect to be working on the site for more than
24 months she is entitled to relief for the cost of travelling from home to the site.
Once her employer enters into a new contract Erica expects to be working on the site for more than 24 months so from that point she is not entitled to relief for her journey from home to the site.

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