Warrant liability - Warrants information (Details) $ in Thousands						9 Months Ended
	May 28, 2019 shares	Mar. 07, 2019	Dec. 20, 2018 shares	Oct. 27, 2017 £ / shares shares	May 26, 2017 £ / shares shares	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 £ / shares
Class of Warrant or Right [Line Items]
Exercise price of warrants | £ / shares								£ 0.01
Number of shares issued for warrant exercise	171,480
Remaining warrants exercisable	80,000
Number of shares into which remaining warrants is exercisable	114,320
Change in fair value of warrants | $						$ 5,381	$ 1,472
Other expenses
Class of Warrant or Right [Line Items]
Change in fair value of warrants | $						$ 100
Series A Preferred Shares
Class of Warrant or Right [Line Items]
Numbers of warrants issued					200,000
Exercise price of warrants | £ / shares					£ 0.01
Warrants exercisable term					10 years
Warrants expiration term following an IPO or exit					12 months
Number of warrants exercised	120,000
Series B1 Preferred Shares
Class of Warrant or Right [Line Items]
Numbers of warrants issued			194,911	115,384	627,903
Exercise price of warrants | £ / shares				£ 0.01	£ 0.01
Number of shares issued for warrant exercise	531,077
Number of shares issued (in shares)				384,615	3,562,583
Share price (in dollars per share) | £ / shares					£ 11.2278
Number of warrants surrendered			194,911
Percentage of warrants exercisable in conjunction with the IPO		50.00%
Percentage of warrants to expire in conjunction with the IPO		50.00%