In the final line of your question you use the words "full time position". I assume this is a slip, and you meant 'permanent position'? - in the sense that your contracted hours are no different between the two jobs?
Custom and practice does have some standing but it is in terms of an implied term of your contract. By its nature, this means that it is not written down (it is 'implied' by action [or inaction] by the employer. You don't have an implied term if you have some documentation (letter) from the employer stating that 'you will be employed on secondment as an X for a period of at least 3 months and until advised otherwise' (or similar wording). It would be interesting to know what the relevant form of words used by your employer was.
To be honest, from the point of view of someone involved in HR over a period of time, it stinks that you can be left so long in limbo, but I think that it is just poor management practice rather than a legal inadequacy. If you've been doing the job at least satisfactorily for so long, it galls that you have to apply for the post against other newcomers. Secondments are supposed to be temporary or development opportunities - designed to demonstrate that the individual could grow into the job. But that is a 'good practice' comment rather than a legal one since your route to resolution would be through Employment Tribunal and it is hard to see how you could get to ET except following a process of being unsuccessful in the appointment, returning to your 'old' job, resigning then claiming constructive dismissal. Not a route one would recommend.