I think the Navpart of the RNS510 is poor TBH , it wont let you put in a 6 didget postcode drives me bloody insane
That is what I initially thought when I went from a TomTom to the RNS510. However I've found it really doesn't make much difference to me to be honest.
A full 6 digit postcode will only lead you to a choice of around 30 houses (at least it does for my postcode) you still need the house number.
A 4 digit postcode brings up all the possible roads that cover that postcode. Click on the road and then the house number!!
To me its only a couple more clicks.
Granted if you only have the postcode and its a business / shop you are looking for then it can be a pain without being able to enter the full 6 digit code.