Frankly I wouldn't go buying any expensive indoor aerial solutions. They are usually a waste of money.
I would go for a decent, inexpensive, Japanese FM tuner with excellent sensitivity & selectivity (and good rejection of spurious signals) and a mono switch.
This sort of tuner (usually designed with the Americans and their distant FM stations in mind) would be far better than the Naim at pulling in a decent signal from an indoor ribbon aerial.
Here is an example for £40 that i'd snap up if I were you (given that it also looks gorgeous in a retro 1970s sort of way) and looks well looked after ...
clicky
If - as you say - reception is pretty good where you are, then a simple 75 Ohm ribbon aerial (get one at Maplin) will probably be ok. Make sure you actually put the 'dipole' of the ribbon aerial (the horizontal of the 'T' shape) up quite high in the room and affix it to something like a picture rail. (Opposite a window is a good place if the transmitter is somewhere in that direction.)
Maplin FM ribbon aerial for £6.99.
(Don't stuff it down the back of a cabinet like most people used to!)
All the other 'amplified' / 'powered' indoor aerial solutions are usually b#####s for the money they cost (often more than a decent tuner from ebay) and invariably end up just amplifying a signal that was poor to begin with.
Experiment with placement of the ribbon aerial (depends largely on which direction the transmitter is relative to your home) and good luck.
Sell the Naim though. You are never going to know what it is capable of without a decent rooftop aerial.