Convertible preferred shares (Details) $ / shares in Units, $ in Thousands						3 Months Ended	9 Months Ended
	May 28, 2019 $ / shares shares	Jan. 03, 2019 USD ($) shares	Dec. 20, 2018 USD ($) shares	Oct. 27, 2017 USD ($) shares	May 26, 2017 USD ($) shares	Jun. 30, 2019 shares	Sep. 30, 2019 USD ($)	Jan. 03, 2019 £ / shares	Dec. 20, 2018 £ / shares	Oct. 27, 2017 £ / shares	May 26, 2017 £ / shares
Convertible preferred shares
Gross proceeds from issues of shares | $							$ 1,334
IPO
Convertible preferred shares
Number of shares issued (in shares)	4,333,333
Share price (in dollars per share) | $ / shares	$ 14.00
Ordinary Shares
Convertible preferred shares
Number of shares issued (in shares)						4,637,666
Conversion of convertible preferred shares to ordinary shares (in shares)						11,647,529
Ordinary Shares | IPO
Convertible preferred shares
Conversion of convertible preferred shares to ordinary shares (in shares)	11,647,529
Conversion ratio	0.6997
Series B1 Convertible Preferred Shares
Convertible preferred shares
Number of shares issued (in shares)				384,615	3,562,583
Share price (in dollars per share) | £ / shares										£ 13.00	£ 11.2278
Gross proceeds from issues of shares | $				$ 6,600	$ 51,900
Numbers of warrants issued			194,911	743,287
Number of warrants surrendered			194,911
Series B2 Convertible Preferred Shares
Convertible preferred shares
Number of shares issued (in shares)		80,385	1,323,248
Share price (in dollars per share) | £ / shares								£ 15.55	£ 15.55
Gross proceeds from issues of shares | $		$ 1,600	$ 26,100