Is this a wheel weight and have my others fallen off as this is the only one?

yes it is and the others can be on in side the wheel rim stuck on

Im going to assume they’re missing as surely the rest should be on the outside too? I wonder if this is why my car won’t keep an alignment? 

Wheel balancing and wheel alignment are two very different things.

That wheel may only need one weight, in a perfect world they wouldn't need any at all but most wheels need one or two.  Wheels with loads of weights on have usually been damaged.

Also as above, most wheel weights are stuck on the inside of alloys...how anyone can hammer them on the face of an alloy like that and believe it looks fine I really don't know.

 

47 minutes ago, dtulip8 said:

surely the rest should be on the outside too

No, as Tom says, they should be self-adhesive types fixed on the inside. This style of balance weight is only really for use on steel wheels.

when i balance alloy wheels , i put self adhesive on the inner side for the outer weight and depending on alloy inner rim edge  a knock on weight ( when you cant see it ) or adhesive if there is no suitable rim edge. but i see alot where there weights have been put on the outer rim edge. not a nice look. 

as for your other wheels , there may not have been the need for an outer weight when balancing

Back in the days original Ford alloy wheels came from the factory with balancing weights hammered onto the inner and outer rim edges.

These days most alloy wheels do no longer have rim edges so these wheels can only be balanced with adhesive wheel weights.

Whether the wheels have rim edges or not I always balance my wheels with adhesive wheel weights that are installed (alsmost invisible) behind the spokes. I personally prefer the old lead wheel weights. Because of environmental reasons wheel weights are made of zinc instead of lead. Zinc weights are much larger than the old lead weights. 

I'm sure there used to be ones on at least 2 other wheels, perhaps I'm going mad. Going in to the garage on Monday for a thorough look over its suspension to find out why the alignment is so shonky. 

Have you had your tyres changed? when you have new tyres it is usual to have previous weights removed as they will not be in correct position any longer and might not be needed at all. But I can't see any connection of the wheel balance weights to the alignment

Good point isetta, I had the fronts changed a few months ago. That should explain it then!