Adding Kerb appeal in February?

We put our house on the market last week but there's been much less interest in it than we hoped/expected. It does seem priced about right as far as we're aware. So far, we've had one 'proceedable' viewing (they came, they spent 45 mins, they disappeared with no feedback given), one 'non-proceedable' which was cancelled, and have another non-proceedable viewing booked for next week.
I know this is generally a quiet time of year but I think our house should be doing better. Imo it doesn't have enough kerb appeal - link to pic here
[url] https://www.dropbox.com/s/j1a0lwf2hely2ye/2018-02-06 13.10.35.jpg?dl=0[/url]
There isn't much scope to do any of the usual front garden things because there *is* no front garden; the front door opens straight onto the street, but I think it looks a bit drab, even though the inside of the house is not. The window ledges are shallow and they slope so we can't have window boxes.
It's in a quiet, safe, pleasant conservation area and we are happy with the inside pics taken by the agent, but I can't help thinking the outside pic of the house may be putting people off. I usually put some seasonal plants outside, as do other neighbours in our little terrace, but as it's February - and very cold as we all know - there's little or no chance of having nice colourful flowers (which provide a good contrast to the black.)
At the moment we have a small olive tree and a dormant climber in containers outside the door but these aren't doing anything to brighten the appearance.
Please could anyone give me some pointers to help me brighten it up a bit? Or am I best being a bit more patient and waiting til spring?