https://www.slab.org.uk/guidance/outlays-for-expert-witnesses-cited-to-attend-court/
An expert witness is one who has acquired, by the achievement of a particular qualification and/or through substantial practical experience, specialist knowledge and skill in a particular field.
It is worth noting that an expert witness is distinct from a professional witness because:
You should contact our applications department if you wish to clarify the category of your witness, as sanction will be required for an expert witness.
Where you have obtained our authority to employ an expert witness, this will also cover the expert’s attendance at any proof. However, if the cost of attending the proof is likely to exceed £2,000, our prior approval must be sought for unusually large expenditure. The rates payable for the attendance at court will be at the same levels as those for preparation of the report. We will not pay a higher hourly rate, unless this has been specifically authorised by us in advance of the court attendance.
In order to assess a claim, we need to know:
General rules for witness expenses: