Read Revenue Leaflet IT54, which explains the ruling on this type of Claim.Technically, if you are travelling to the same Client 5 days a week for more than a few months, then the revenue would consider this to be your "Normal Place of Work" - thus disallowing the mileage expense claim.
You need to be performing substantive duties at your own Company Address for this mileage to be allowable, i.e. working from home 2 days a week doing research & development, for example, and Claim mileage (and subsistence for 3 days).
Some contractors claim the full amount, others for maybe a 3-day week, most claim something, and your case could need to be argued in an audit I would say.
Regards,
Mac.