Hi Louby. There are a lot of threads on the forum signposting people to useful resources. If you use the search facility in the top right hand side from the main careers page and search for "mechanical" or "psychometrics" you may find them. Can't remember the exact books I used but I know I've flagged them up on here before. One was How2Become A Train Driver but there were others that were equally if not more useful.
As someone who struggles with mechanics, of most help to me was a friend who has a good level of knowledge using household objects to help demonstrate levers, pulleys, pivots etc. etc. It helped me to visualise what happens if, for example, a cog turns one way and you need to work out which way another turns.
One thing I will say is that even a fitter from Leeds railway station struggled with the mechanics test, as did all of those who got through.
Hope this helps, sorry if it doesn't make much sense!