Quote: I also have the same yellowing leaves which drop.

This sounds that the plant is either pot bound and the pot is void of any nutrition and / or the pot is waterlogged, or a permutation of all these things.

Is the pot slightly off the deck? If not this may bre interfering with the drainage!

You say the pot/tree is too big to repot!

Usually what you do when faced with this situation is; dig the top two or three inches of old compost out and replace it with new.

If drainage is the problem I would consider ( but then that is me) tipping the pot and tree on its side and clearing the drainage holes then uprighting it again on to bricks or purpose made pot stools so that water does not settle under the pot.

If you do decide to do this do not hold the tree to right it, hold on to the pot, say one person on one side pulling and someone else on the other side lifting andpushing.

If you use the tree for leverage you might snap it!

Quote 2; It is now mid April should I prune it and by how much.

What you have to be careful with is what branches you prune out as the fruit is produced on one year old wood, meaning, you can remove older branches than this to allow new branches to form for next years fruit.

Quote; I generally use a tomato fertiliser two or three times a year? Is this correct. I had a good crop of olives last year.

Yes as you have found from previous experience.

This link might be of assistance;http://www.thegardenersalmanac.co.uk/Data/Olives/Olives.htm

Many thanks. The pot is in a shallow saucer which is close fitting (about 1cm gap all around) so it holds very little water and I doubt it gets waterlogged! i put plenty of crocks in the base to allow drainage when it was repotted, I will though get some assistance with muscle and put under some lifts. The yellowing tends to be annually around late July.
I will have a go at a thinning rather than a prune - hopefully this will restrict root growth!

Quote; I doubt it gets waterlogged! i put plenty of crocks in the base to allow drainage when it was repotted,

I will though get some assistance with muscle and put under some lifts.

Reply; Sounds like you won't need extra muscle if you are happy with the drainage situation, don't bother with what I said in the previous posting.

Just remove the top 2"-3" of existing compost and replace with say some JI3 or its equivalent and that should be fine.

Quote;The yellowing tends to be annually around late July.

Reply; This sounds as if it has used up its nutrient reserve by then! Just rake in a top dressing of fertilser every two months e.g.

April; replace top 2"-3" of compost.

June; rake in a top dressing of fertilser

August; rake in a top dressing of fertilser and this should last out till the following April

In between give it the high potash feed as you have been doing and that should suffice

Then follow the same programme each year thereafter.

Quote; I will have a go at a thinning rather than a prune - hopefully this will restrict root growth!

It won't, but it will allow new growth to form unhindered for next years fruit.

If the pot is sitting in a shallow saucer then I would question whether there is good drainage. I would go with Teegee's first suggestion and remove the saucer and raise up on feet.

i agree with moon grower infact it sounds if possible like this plant needs getting put in the ground . easier said than done i know sometimes .