Share option scheme and Restricted Stock Units - Narrative (Details) - GBP (£) £ in Millions	3 Months Ended
Apr. 30, 2018	Apr. 30, 2017
Disclosure Of Share-Based Payment Arrangements [Abstract]
Number of share options capable of vesting	10,009,284
Share-based payment expense	£ 0.5	£ 0.3

The amount of expense from share-based payment transactions with employees.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_112_c&doctype=Standard
-URIDate 2017-03-09

Name:	ifrs-full_ExpenseFromSharebasedPaymentTransactionsWithEmployees
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration