Would a masters in maths at a much lower ranked university be a good preparation for a masters course at a very top university? I have finished my mathematics undergraduate degree in the UK at a low ranked university. I just missed out on a 2:2. 
I struggled with modules such as statistics and Operations research but excelled at pure maths and applied maths modules. 

I have miraculously still been accepted by a university to study for MSc in maths. 
This university is not part of the Russell group universities but does do better in the league tables than some of the universities that are part of the Russell Group.

Out of fun I was researching the masters course in maths at Cambridge.
In the handbook they mentioned something interesting.
They said that if your first degree is not up to scratch then you may take the second and third year at Cambridge to bring yourself up to the right level.

After contemplating this, I am thinking about doing a second masters in maths but with the aim of doing it at Cambridge. 
Off course I will need a very high distinction to do this.

I was wondering in general, would a masters in maths at a much lower ranked university be a good preparation for a masters course at a very top university?
They said that if your first degree is not up to scratch then you may take the second and third year at Cambridge to bring yourself up to the right level.


Are you talking about Körner's unofficial guide to Part III?  He says "If you are a very able student whose first degree course is not up to the standard required for Part III, it may make sense to apply to become an 'Affiliated Student' taking the second and third year of the Cambridge mathematics course to obtain the Cambridge first degree."  This is closest statement I'm aware of to what you mention, but it means something different.  It's talking about repeating undergraduate studies (when the applicant did very well in courses that are insufficient preparation for Part III), rather than repeating a master's program.  Furthermore, admission as an affiliated student at Cambridge requires first or upper second class honours.

I'm not at Cambridge and can't offer definitive guidance about what they will or will not allow.  However, I'm skeptical about the idea of studying for a second master's degree.  You shouldn't plan on this unless you have a clear indication that Cambridge will consider it.  I'd bet that the chances are low.