If the professional considers that, for any reason, the Letter of Claim does not include all the requisite information as listed in point 3 above, they should, as soon as reasonably practicable, let you know why and identify the further information they reasonably require.
The professional has three months from the date of the Letter of Acknowledgment to investigate and respond to the Letter of Claim. This is done by the provision of a Letter of Response and/or a Letter of Settlement (see point 5 below).
If the professional has difficulty in complying with the three month time period, they should explain the problem to you as soon as possible and, in any event, as long as possible before the end of the three month period.
All parties should supply promptly, at this stage and throughout, whatever relevant information or documentation is reasonably requested.
If the professional intends to claim against someone who is not currently a party to the dispute, that third party should be identified to you in writing as soon as possible.
See here for Step Five - Letter of Response and Letter of Settlement