My acer has rust and is dying at the end of the leaves, any ideas why please

I am assuming it is a Japanese acer, as you say it is in a pot. These lovely plants are difficult to grow unless given the right cultural conditions. They are woodand plants, which don't like strong winds, strong sun, frost, and prefer an acid growing medium and damp soil. They are fairly quick to show signs of distress, and one way this happens is that the leaves crisp up. If you can move your plant to a shaded, sheltered position, give it a good drink at the roots, mulch it with an ericaceous compost, it might improve. Hope this helps.

And maybe you should treat the rust with an appropriate spray for fungal infections.

I should add that the purple leaved varieties require some sunshine to keep their colour, but not strong sun.

I grow about 12-14 Japanese maples in pots, and my bible is: Japanese Maples by JD Vertrees with Peter Gregory; It states that whenever water lost from leaves faster than the roots can take up, brown dead patches spread through the leaf tips. Invironmental factors like scorcing sun, , drought & drying winds can also cause this problem. Usually the plant is not lost, just the appearance spoilt.
So like Louloubelle said prevent these conditions. good mulching, reduceprolong drying too much sun, 7 water early or late in the day. You did not state the name of your Maple??

If you can find the source of the problem like Doon and Louloubelle have suggested then you will not need to spray. Using the spray may only give you an intermittent cure, but if the plant is not getting the proper care and conditions it requires the problem will reoccur. Trim off the unsightly growth and maybe repot it, the soil may be the problem. Use an ericaceous compost with some John Innes No3 (loam) and add a few slow release acidic feed. Give it a good water and place it in a sheltered spot in partial shade.