Are you still working for the trust? If so, at least a proportion of the payment will be taxable for the reasons Mrs P outlines. However, you may be able to argue that the gift should be taxed on a pro-rata basis accross all the years you have worked, but it is likely to be a tough argument to make.
If you have not worked for the Trust in the UK then you may be able to argue that it relates to previous offshore activities.
You should disclose it to the Revenue and seek a ruling. Given they lean towards taxing, you should consider getting professional advice first so the best arguments can be presented.