A few additional observations on a most interesting question.
The reason the solicitors want a statement is to assess what a witness's likely evidence will be.
Thereafter, they may need to interview the witness to assess their evidence and witness potential.
From that they work out if the evidence is going be helpful, harmful or of no evidentiary value.
If helpful they can serve the witness with a witness summons which compels attendance at court.
Being a compellable witness means that, if served with a witness summons, you must attend court.
Failure to do so is a refusal to comply with a court order.
If a witness has potential legal exposure of a criminal nature they can decline to answer a question on the grounds that to do so might tend to incriminate them. Even where a witness is giving evidence in a civil case some answers could be incriminating elsewhere.
If there is no potential criminal exposure an answer will be required.
One hazard in this type of situation is that of being declared a hostile witness.
A witness can be called to court by one side as being one of "their" witnesses. If that witness by their answers or non-responses appears to be antipathetic or even obstructive to the side that called them counsel may apply to the trial judge to have that witness declared a hostile witness.
This means that the side that called that witness may now cross-examine them.
Generally, you cannot cross-examine your own witnesses.
Another little gem lurks for the reluctant witness. If a person, not being a party to an action, has possession or control of information, documents or even objects of evidentiary relevance and importance such a person might be the subject of an order for discovery. This means that the person against whom discovery has been granted must yield up whatever they have in accordance with the terms of the discovery order.
BTW 1, a refusal to provide a statement to the solicitors does not have to be explained or justified.
BTW 2, witnesses are entitled to receive reimbursement of any expenses incurred directly in coming to court. The witness applies to the side that called them.