Of all the Vauxhall Astra GTCs you can buy at the time of writing, this is the one that is expected to be the most popular in the UK. It's also the most expensive. Which just goes to show something or other.It's the most expensive partly because it has the 163bhp two-litre turbo diesel engine which is second only in power output to the 178bhp turbo petrol unit (two versions each of a 1.7 diesel and a 1.4 petrol are also available) and partly because it's in the SRi rather than the cheaper Sport trim level.SRi models cost about £1500 more than Sports, and for that you get 18" alloy wheels, air-conditioning, active headrests, six airbags, ESP, a USB connection, DAB digital radio, cruise control and daytime running lights.You can go further, of course. The test car had several options which added an extra £2545 to its price, namely 19" wheels, a mobile phone system with Bluetooth connectivity, LED rear lights, FlexRide adaptive damping and the Navi 600 system whose features include satellite naviation, a CD/MP3 player, seven speakers, auxiliary input, an SD card slot, a shark's fin aerial (not actually made from a shark's fin) and so on.No matter what GTC you buy, you get a very smart shape - generally agreed, as far as I can tell, to be the most handsome yet devised for any coupé-style Astra - more interesting interior design than Vauxhall seemed capable of in the very recent past, a surprising amount of luggage space, an only middling sense of quality (better than the Corsa, not as good as the Insignia) and an impressive driving experience.On that last point, the GTC doesn't feel tremendously sporty in any of its current forms, though the forthcoming VXR surely will do. This is, however, a car which responds quickly to the major controls and can corner quickly with excellent balance. During the on-track section of the GTC's media launch event earlier this year the limiting factor of the 1.6 petrol turbo was understeer on slow corners and oversteer on fast ones (the diesel, with its heavier engine up front, would probably tend towards understeer in both situations) but in each case the ESP system cut in well before anything distressing happened. On public roads you're not going to encounter either unless you are a certifiable loony.This is the moment to bring two of the test car's options back into the conversation. I would definitely go for FlexRide, even though the normal, Sport and Tour modes don't produce very different results - different enough, nonetheless, that it's more worth your while experimenting with them than it is in the Insignia I drove recently, which seemed to work best in the standard setting.I would not, however, spend £565 on the 19" wheels and lower-profile tyres, since - call me old-fashioned - I care more about how well a car rides than how good it looks.Furthermore, if I were to buy an Astra GTC I'd be inclined to choose a petrol version rather than this 2.0 CDTi, especially if I didn't anticipate racking up a large mileage each year. The diesel engine is quite loud by modern standards, and its weight makes the car less poised than other cars in the range. Then again, it might only take better sound-deadening and a slight rethink of the front suspension settings for this GTC to deserve the popularity which Vauxhall believes it will have. Engine 1956cc, 4 cylinders Power 163bhp Transmission 6-speed manual Fuel/CO2 58.9mpg / 127g/km Acceleration 0-60mph: 8.4 seconds Top speed 131mph Price £22,430 Details correct at publication date