But surely you enter into a contract with the solicitor where they agree to provide certain services in return for the, not insubstantial, fee that you pay them.
If submission of all relevant forms in a timely manner comes within the scope of the services they contract to provide (and let's face it, what do they do apart from submit documents in a timely manner) then I would say they are in breach of their contractual commitment to you.
At the very least they should be obliged to pay up for any costs you incur through their mistakes.
I would try the Citizens Advice Bureau, they used to offer a free legal helpline
Jools
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