You can now buy rhododendrons which are grafted onto a lime tolerant rootstock, which is supposed to enable you to grow them in alkaline conditions. I'm not sure how big the range of available ones is at the moment, but if you Google 'lime tolerant rhododendrons' plenty of sites appear to be selling them - details of height and spread are probably available from sellers.

Thankyou but I already have the rhododendrens so I require some product to put aroun the roots so they will be happy plants can you help

Ah. There's no real solution this problem. If the soil is alkaline, its alkaline, and will need constant correction. You can apply peat around the base, flowers of sulphur may help, otherwise its probably dosing with Sequestrene iron tonic yearly or six monthly. Comes in granules to be applied to the soil, or as a mix for a can to be watered on.

How long have they been planted in the soil?

If not long, would it be possible to lift them, then take out a deep hole and fill that with ericaceous compost before replanting them?

It depends really on the soil. Just how alkaline is it?

It is not that the plants actually require acid, it is more that they need the 'absence' of lime.

So, a neutral soil will do if you keep it topped up with peaty, or acidic feed. Leafmould is very good.

If you have truly limey soil, then get some BIG pots.