which rootstock for small espalier or fan apple tree

forgot to add it is south west facing (gets lovely and hot) and I garden on clay soil which has a ph of about 7

The summer pruning which is used for espaliers, fans and other trained forms means that lots of new (over-vigorous) growth can be removed in summer time and allows quite a lot of tolerance in how big you let it get.

The rootstock MM106 is generally considered to be the best for espaliers, but I would add that it can be a very fussy rootstock which can fail to thrive in low-rainfall regions, yet can suffer from root rots in heavy rainfall regions.
M26 is more predictable in a variety of locations. In good soil it is about 20-25% smaller than MM106, but in certain poor soils MM106 struggles and can actually be smaller than M26.

In some instances, you may need an even stronger rootstock, since despite what "the books" say, the fruiting variety on top and its vigour can significantly affect how vigorous the tree is.
If you bought (say) a Cox and Bramley on the same rootstock, the Bramley would probably get to almost twice the size and take much longer to start fruiting.
But I recommend avoiding the common varieties because they tend to be disease-prone unless regularly sprayed - and Cox, Bramley, Gala, Braeburn, Golden Delicious are available in shops at cheap prices anyway, while some are not suitable for most parts of the UK (being too cool and dull).
Often, the best varieties for an area are those rare old varieties which originate from nearby, or from areas with similar soil and climate.

Also note that part-tip-bearers are not ideal for training into restricted forms because of complicated pruning.

A guide to the relative vigour of varieties and rootstocks is here:
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wow! it is more complicated than I thought. I live in Blackpool. Contrary to populare belief, Blackpool is not as rainy as you might think and receives a good deal less rain than say Preston. My soil is very good but is clay and it will be placed in a very sunny and sheltered spot. Please can you recommend varieties. Very many thanks Helen

No apple variety is perfect, so it is always a case of finding one with the most characteristics to suit your needs.

Some questions to head in the right direction:

Do you want eaters or cookers?

Do you want dual-purpose, but with the likelihood that they won't be the best cookers nor the best eaters.

Do you want the apples to ripen in August, or do you want them to ripen later?

Do you want ultra-late-keepers which ripen in October and can be stored in a box in the shed?

Do the fruits needs to be attractive or preferred to be any particular colour? Do you find russet apples attractive?

Would you like large fruits, medium fruits, small fruits - or not bothered?

How tough will the tree need to be - will you be needing it to fend for itself?

Do you plan to buy from a local garden centre or nursery and have to choose whatever they have available (which is often quite a limited range of common varieties)?

EDIT:
How common are other apple trees in your area? Will there be a suitable pollinator somewhere in a nearby garden, or will you need a self-fertile variety?

Or if you just want a list of varieties which have reasonable all-round disease resistance (ideally, you would choose a variety with strong resistance to diseases most common in your area - and this varies by region; you may need to pay attention to scab and canker).
They are also fairly trouble-free, reasonably easily pollinated and should ripen properly in most UK regions:

Egremont Russet
Crawley Beauty
Laxton's Epicure
Bountiful
Howgate Wonder

Also varieties which may need a relatively mild location to ripen properly:
Spartan
Winston
Brownlees Russet

Also some varieties which are fairly disease resistant, but also fairly vigorous and/or part-tip bearing and/or triploid or low-fertility (so need a pollinator in a nearby garden):

Belle de Boskoop
Hambledon Deux Ans
Edward VII
Alfriston