The short answer is: no (not at the pre-action stage i.e. before the matter goes to court).
The pre-action stage is cost-free and there is no financial risk to your client.
The letter before action is simply asking the lawyers for the decision maker to look at the decision or action/inaction again (because the decision has been reached in the wrong way etc.).
In straightforward cases where there has been a clear failure to apply the law correctly or simply a failure to act, once the public body's lawyers have looked at it they will simply do what you are asking them to avoid the risk of you taking them to court and there being a negative judgment against them.
If they look at it again and do not change it, the decision maker will write to let you know, and you and your client can then decide if you want to pursue it to court.
If your client wants to pursue it, you will then need to refer the matter to a solicitor. (Challenging public bodies in the courts can be done by individuals representing themselves but they are immediately at a disadvantage as the public body will always be represented in a judicial review claim). Here is a network of solicitors you can make referrals to.