Question
please could you tell me if i can mix 12 month old sheep manure with potting mix to plant citrus trees in pots

Answer
You can use it. I would mix it 30-35% manure with 65-70% potting mix. If the potting mix contain fertilizer there is no reason to add more fertilizer before planting. If not, I would still add some high nitrogen fertilizer (10-10-10, or 20-20-20).

Nitrogen in sheep manure takes time to mineralize and become available and only about 25% of the nutrients in the sheep manure will be available in the first year. After the first year, more of the nitrogen will be available.