Wow, the OP asked for practical suggestions, but seems to have received a diatribe of peoples personal views on the London property market!
If you've not had any viewings in 4 weeks, the first thing to do is ask your agent why. After all, this is what you are paying them for.
It would be interesting to understand their rational for the price given the sold price of No. 20. Do they feel the market has moved that much since the 3rd March? Zoopla (notoriously unreliable) is suggesting there has only been a 0.6% increase in that same time frame. They should be able to explain their view of the market vs. this. Do they sound credible? Do you believe what they are saying?
Ask your agent to show you the properties they feel are your competition. How does yours stack up on prices vs. condition?
How many other sales have your agent had in the recent months - is the market just stagnant at the moment?
The housing market typically cools off towards the end of the year as people start thinking about Christmas. If you're not in a rush, set your-self a hard time line to try and sell then potentially try again in the spring when the market is typically more boyant.
Another practical step is to ask other agents on their view of the price or why you might not be getting viewings.
Think about what you would accept for the property and consider an on-line estate agency. I'm guessing your on around a 1.2% commission, so you could probably save 10k by using an on-line agent.
I wouldn't bother with any interior decoration or changes. If you are worried that that's putting people off, simply take the interior photo's offline and get people through the door.