If a local planning authority decides that your application is invalid, it should let you know the reasons in writing, or if there is a simple solution, telephone or email you. More information is available on the Planning Portal, under invalid applications. Normally, providing the additional information required is fairly straightforward, and is the quickest way of getting your application validated and the clock ticking for a decision.
However, if you can’t resolve your dispute with the local authority there is a procedure set out in Article 12 of the Development Management Procedure Order to resolve such disputes. The applicant can serve a notice on the planning authority saying why the additional information which has been requested is considered unreasonable and requesting that the requirement be waived. The planning authority can then serve a notice that it no longer requires the information called "a validation notice" or one saying the information is still required, "a non-validation notice". The Planning Practice technical guidance website provides more detail on delays in the validation of an application (paragraph 049).
After the statutory time period for determining the application has expired, the applicant can appeal to the Planning Inspectorate against non-determination (this is basically the same procedure as if the application has been refused). In considering such an appeal the Inspector will consider both the dispute regarding invalidity and the merits of the application itself (although if the Inspector agrees with the local planning authority that the application was invalid, the appeal will be dismissed).