On paper, some ideas seem great. For instance, you can see how the following would seem like a good idea if you were Mercedes-Benz: take one, front-wheel drive, A-Class platform. Give it a footprint and space inside that’s roughly the same as a C-Class saloon and then mix together with a body that uses the bigger, much more exotic four-door coupe style of the CLS. Et voila, it’s the Merc everyone should want. It costs less than a C-Class, but offers a lot more pizazz than most three-box saloons. A sure fire winner? That’s the theory anyway.

Opening the door, we’re immediately not so sure. The CLA uses the A-class’s dashboard architecture and much of its interior is similar. Yet while in the A-class it felt generally fine, in the slightly more premium positioned CLA, it feels a little cheap and plasticky. It’s not bad, but it’s not exactly signing the cheques the exterior purports to write, put it that way.

And is it our memory, or are we sitting higher than we were in the A-Class, even with the seat on its lowest setting? You seem to be sitting on the car, rather than in it. It could be to do with this car’s (overly) extensive spec, which includes electric seats – notorious for not going as low as their manually moving counterparts due to all the gubbins that needs to sit under your bum.

Merc’s COMAND interface is a two-grand option that you must choose, however. Here it’s presented to you via a 7-inch tablet-ike screen – nice apart from when you glance behind it and see how it’s attached to the dash. If you don’t option COMAND, you do still get a tablet screen, albeit a 5.8-inch job. But it’s the feature set you lose that you’ll really miss as opposed to the physical size of the screen. Once acquainted, it’s just as easy to toggle through and pull up vast amounts of information with this interface as it is on any of the company’s bigger machinery.

The seats - while set too high for us - are otherwise great. Most models come with big wing-back units as standard, which have the headrest wrapped into the body of the seat. And while we find the exterior design really quite fussy and busy, it’s an aesthetic we’re sure will appeal to many. It certainly gives the CLA an edge in this class if you’re looking for something different.

But it’s out on the road where the CLA truly starts to fall down. Mercedes’s "220" cdi engine always seems to have plenty of punch and is pretty linear and coupled to the 7-gear automatic box it makes for easy progress. But it has always been a slightly agricultural-sounding unit - in truth, a criticism of most modern direct-injection diesels. Here, subjectively, it’s an ever-present background mill, and is certainly more intrusive than we remember it being in the C-class.

Our biggest gripe is with the ride. We’re used to modern cars riding relatively firmly. The atrocious, pot-holed state of many UK roads still shocks many car company chassis development guys we speak to and it shows when you drive their cars here. Pair that with the designer and customer-driven trend for bigger wheels and a "sportier" experience and you’ve a recipe for a bone-rattling time. Which is the context for telling you that even on (not the biggest available size) 18-inch wheels of our CLA, it is at times uncomfortably hard-riding. It never truly settles, jiggling around on anything but perfect surfaces. Potholes and ridges can really crash through the structure too. Shame. Mercs used to be bastions of comfort.

On the positive side, you can expect great frugality from all the diesel engines offered (there are three) and the flip side of the ride situation is that it offers really impressive levels of grip and quite neutral handling for a front-wheel drive car. The steering feel is meaty and pleasant, too. Plus, you can option all of the leading safety-related kit (Distronic, Lane-Keep Assist and so on) that is available on the firm’s bigger cars.

It’s a little cheaper than the equivalent C-Class too (£29,355 for this model). Combine that with the low CO2 engines and it should make a decent option if you’re a company car driver and it’s aimed squarely at the Audi A3 TDI crowd.

We expected the CLA to be great, a car that shook up a couple of sectors because it’s effectively invented a new one. The reality is that we’d say test drive one and see what you think, but for our money the C-Class is a nicer place to sit and better to drive.

The caveat, of course, is that we were able to drive only this car – the C220 CDi in Sport spec. Somewhere in the CLA range, we suspect, lies a spec and engine combination which might show the car in a more favourable light.

Until we’ve driven a few more variants, we’d suggest that if you’re being swung by the CLA’s looks, stay away from the options list (our car was £40,845 as specced) and keep the wheels as small as you can cope with, aesthetically.