So, your school just has a crap system. That sucks, but there's not much you can do about it - don't worry, I feel your pain, hahah.
There are a lot of differences, but there are also a lot of similarities.
Firstly, at least 60% of what you do in methods is relevant for specialist, but only 20% of specialist is relevant for methods. However, you need methods for specialist - so, if you do specialist, you're doing methods anyway.
Further is different, and it depends on what modules your school teaches. If they teach geometry, this is assumed knowledge for specialist, so there's some nice overlap there. If they teach graphs and relations, this is taught in the methods curriculum, so there's nice overlap there. If they do Matrices, most of this is assumed for methods (only exception is transition matrices which has recently been taken out of methods), so there's some nice overlap there. If they do discrete maths, there is overlap in the NEW spec 1/2 (so, you'll probably miss out there).
The last part of Further is a core unit, stats and business maths - there is some very small overlap with that in methods, but most of it will be new, and the core module covers 60% of what you need to know.
So, there is some overlap, but it's still different between all three. Don't expect to magically do well in further just because you have methods, but also don't expect to even do specialist without methods (I hope I've made it clear that you need methods for spec
)
The way the subjects are traditionally viewed, further is easy maths you do if you don't want to ditch maths, methods is hard maths that you do because you're good at maths/need it as a pre-req and specialist is insane maths you only do because it has some very nice scaling (third best, to memory).
I also want to reiterate that you'll be doing these subjects with a new study design - that means that what's currently taught in the maths subjects will be different when you do the subjects, so try to seek advice from someone who understands the new study design, and not just people who are currently doing the subjects.
EDIT: Argh, being beaten by Crystal...
Keeping all this up though, because reasons.