(Replace the words in square brackets)

[General Secretary]
General Secretary
Union
Address

Dear [General Secretary]

During the period [date] to [date] I have been subjected to bullying and harassment whilst in the position of [position] at [place of work] in the employ of [employer]. The behaviours exhibited towards me by my employer breach employment, discrimination, contract and health & safety law. The local branch of [union] has chosen to refuse to support me and I now wish to bring these matters to your attention.

I am a member of [union] and I pay monthly subscriptions.

I require representation by [union] in grievance, disciplinary and legal action.

I have been refused support and representation by [union].

In order to take legal action I now have to use the services of a legal representative outside the union.

I believe that [the union] is in breach of contract and I shall recover from [union] all sums incurred.

Yours sincerely,

etc...

If the Union has supported you but dropped you just prior to legal proceedings you might like to think about this:

[Union] has supported me throughout the period [dates] but has dropped my case [x days] before the tribunal. At such short notice the union knows I will not be able to find alternative legal representation, I have no finance for an independent legal action, and my health is severely compromised. [Union] knows that by dropping my case so close to tribunal I have virtually no chance of clearing my name and obtaining justice. In full knowledge of this, [Union's] action is deliberate, intentional and wilful and is therefore tantamount to sabotage.

There's more about the refusal of trade unions to support members in cases of bullying and stress on the Public Sector page.