"Discretionary Trusts" are a category of trust; within that category, there can be wide variation. It is impossible to give any specific assistance without knowing more.
By way of general observation only:
1. There is no point in taking the attitude that "it's my cash", at least not in the sense of suggesting that your wishes\instructions should in some way automatically be followed. A discretionary trust means just that- the trustees usually have a discretion as to when to advance money to the beneficiary; whoever set up this trust did not want you to have control of the capital, at least not at the time the trust was set up. You might not like this, but that is the way it is.
2. There is no audit requirement in relation to the administration of a trust. If asked to guess why the Trustees have refused to get the accounts audited, I would say the most likely reason is that they do not wish to waste trust money.
3. Of course, it is perfectly open to you to engage your own auditor to examine the trust accounts. If the Trustees fail to furnish information (when requested in writing) to you, your solicitor, or your accountant\auditor, then you might well have grounds for complaint.
4. "They seem to be a law on to themselves and accountable to no one." In a sense, as regards the management of the Trust Fund, it is probably true to say that the Trustees are a law unto themselves. But that is the whole idea. If the assets were simply left to you, you would be a law unto yourself as regards the management of your assets. It is not, however, true to suggest that they are accountable to no one. If the Trustees are solicitors or accountants, then they are of course subject to regulation from their professional bodies. In any event, all Trustees are subject to the oversight of the courts. It is always open to you to issue legal proceedings if you feel the trust is being mis-administered. But (and this is a big but) the mere fact that the Trustees of a discretionary trust are not willing to go along with the wishes of a beneficiary doesn't really prove anything. As already stated, the whole reason for having a discretionary trust is to take decision-making power away from the beneficiary and give it to somebody else (the Trustees).