Ctec charger will be absolutely fine.No need to disconnect the battery.Put the charger clips directly onto the battery terminals.

I would switch all "auto functions" off - lights etc etc and for 5 weeks I would suggest the car would be 100% without any trickle charging.

The only thing that should consume any power is the alarm!

A 2 year old battery on a diesel should cope.

I've used a CTEK on various cars without disconnecting the battery, and I leave one on my MX5 throughout the winter when it is used infrequently (that car also has a small battery).

You are correct to say that the ground/black lead (-ve earth) should technically be connected to the frame away from the battery and from fuel pipes. This is to reduce the chance of any sparks when you disconnect (black lead first) igniting any fuel vapour, or any hydrogen in the vicinity of the battery. In practice this is pretty unlikely. It's a simple precaution. In any case I always plug in the charger after connecting the clips, and unplug it before disconnecting. There are no handy unpainted surfaces in the boot of the MX5 where the battery lives so I connect to both terminals.

I'm pretty sure I will have used it on my CRV, when I had it, at some point too.