Business mileage etiquette

[vrsmxtb]

Ok my company has very vague guidelines over claiming business mileage payments in terms of what you measure and from where and I can't get a clear answer from anyone.

I am currently on a secondment which involves me travelling to various sites all over London and the South. I pick up my mileage from the base site I was based in before the secondment. My mileage is all claimed from this base store, unless I am in a hotel for the week.

I'm fairly certain I should be deducting my weekly pre-secondment normal commute from my mileage claim, even though I'm not actually starting or finishing my journey at my base. But if I do the mileage from the base site to the visited site this effectively takes into account the commute, it's just a bonus if my home is closer to the visited site than the base site.

Any ideas on the standards of what is acceptable?