The page offered was good.
Witnesses expenses and allowances: Legal Guidance: The Crown Prosecution Service
There is a PDF to download to fill in, the guidance is:
Ordinary Witnesses
Witnesses may receive compensation for:
travelling expenses
money spent on refreshments and meals
financial loss (e.g. loss of earnings)
other expense incurred (e.g. childcare
NB: There are limits on the amounts payable:
bus and train fares (standard fare) are normally repaid in full;
travel by bicycle is paid at 20p per mile;
travel by motor vehicle is paid at a rate per mile;
a higher rate per mile may be paid if it is necessary, as opposed to just convenient, to use a private motor vehicle;
taxi fares are allowed where this was the only form of transport available or if a witness is disabled or infirm.
set limits apply to refreshments and meals;
maximum daily amounts apply to loss of earnings and other financial losses.
the PDF goes on to explain:
Loss of earnings – self-employed persons (Ordinary Witnesses only)
The maximum amount of earnings lost that may be repaid is:
For an absence from work not exceeding 4 hours
£42.95
For an absence from work exceeding 4 hours
£85.90
Payments to compensate for temporary loss of profits are taxable as receipts of a business. To claim loss of earnings, you must provide some evidence of self-employment (eg business card or letterhead with your name shown, firms stamp, tax or VAT reference, accountant’s letter) and you may be required to provide evidence of your loss. The decision to pay the higher rate is discretionary and it will not be paid automatically.
Motor car: If you use your own motor car, you will be paid 25p for each mile. In addition, if you give a lift to another witness who is required to attend court or other person authorised to accompany you, a further 2p per mile will be paid for the first passenger and 1p per mile for each additional passenger. Exceptionally, if you can show that you had to use your own car instead of public transport (for example,
because there was no public transport, or there was a considerable saving of time and money, or because you are disabled or infirm), then a higher rate of 45p per mile may be paid. You will need to justify using your own motor car to be eligible for the higher rate.
Finally:
Principle
The CPS is responsible for paying allowances and expenses to witnesses who are called to give evidence in prosecutions conducted by the Service.
The CPS aims to pay 100% of correctly completed witness expense claims within 10 working days of receipt.
I think this is the relevant webste for your situation.