TSEM8717 - Trust management expenses: allowable expenses: specific items: accountancy: software for making returns
If the agent has incurred the cost of return software and the trustees have not incurred any, then there is no question of TMEs.
If the agent has charged the trustees for the cost of software and the trustees have effectively incurred that cost, then relief for TMEs may be in point.
If the software licence is for one year only, this is likely to be an annual cost of making an income trust tax return - see TSEM8715.