The Law Society is not the regulator of solicitors any more. You need to approach the Legal Ombudsman or the Solicitors Regulation Authority.

Asking for a complaint to be made in writing does not seem unreasonable to me.

Although, from what you say, it is not entirely clear what the situation is, it appears likely that the SRA has intervened in the practice (which would most likely be a partnership or an LLP, unlikely to be a company) of your solicitor. Intervention is the process by which the SRA take possession of the client files/documents and client/office monies of a practice - essentially, an intervention agent is appointed, they back a van up to the premises, and they take the files, whilst also getting the monies transferred into new accounts which they control.

The entire process is, inevitably, a mess because the intervention agent knows nothing about the practice and has to fire fight for all the clients. Hence, the most urgent will get the first attention. It is not clear from what you say whether or not this practice was acting for you on a sale of a property, or whether they were just holding (in their strong room) the "deeds" to a property which you own and which you are in the process of selling using a different solicitor.

If the former, then you need to contact the SRA and find out who the intervention agent is, or perhaps phone the practice (where there might be a phone re-direction in place to take you to the intervention agent, or an answering message giving you their details). Then contact the intervention agent and tell them what the practice was supposed to be doing for you. If it is a completion tomorrow, then the internveion agent will likely deal with it. If you are at an earlier stage of the process the intervention agent will ask you to instruct some new solicitors and arrange for the file (and any client money - as long as there is enough in the client account to go round everyone) to be sent to your new solicitor asap.

If the latter then, understandably, you will be of a lower order of importance for the intervention agent. But, still you can and should find out who they are and ask for them to look for your "deeds" and send them to you (or to your other solicitor) - although as BV says it is likely that your property is registered and that the "deeds" are of historical interest only (or perhaps relevnat if the purchaser's solicitor is a pain in the backside).