Can I get a solicitor to certify my documents rather than waiting for the Nationality checking service.I have contacted most of the local authorities in London and environs and the earliest appointment they have is in mid october.Is there a disadvantage in a solicitor certifying my documents?

You do not need to certify any documents if you are sending them Original documents.  NCS is only used for reducing time and they are familiar with the procedure and can certify documents for you and returns you teh original. If you cannot find NCS and don't want to use their services, Home office provides  this service free but it takes time(In my experience)

If you want to go directly to Home office, send all documents to them and they will make copies and varify them with originals and send you back original documents.

NCS is there to provide you service and expedite your process, you don't need to use them. If you don't have a good reason(Traveling soon), Home Office Requires you to send them the original documents.  It is very clear in the Guidance notes.

I will still say that the NCS charges are well worth and if you are getting late appointment, it is wise to use their services.