Summary of significant accounting policies - Changes in accounting policy and disclosures (Details) £ in Millions	12 Months Ended
Jun. 30, 2018 item	Jul. 01, 2018 GBP (£)
Disclosure of expected impact of initial application of new standards or interpretations [line items]
Number of new standards, amendments or interpretations, effective for the first time, that had material impact \| item	0
Forecast \| Increase (decrease) due to application of IFRS 15
Disclosure of expected impact of initial application of new standards or interpretations [line items]
Net assets \| £		£ 1.8

The amount of assets less the amount of liabilities.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Name IFRS
-Number 1
-IssueDate 2017-01-01
-Paragraph IG63
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=1&date=2017-03-01&anchor=para_IG63&doctype=Implementation%20Guidance
-URIDate 2017-03-09

Reference 2: 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_NetAssetsLiabilities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant