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Financial instruments at fair value (Tables)
12 Months Ended
Mar. 31, 2019
Fair Value Disclosures [Abstract]  
Fair value of assets
   March 31, 2018  March 31, 2019
$ in thousands                        
   Level 1  Level 2  Level 3  Total  Level 1  Level 2  Level 3  Total
Equity investments   78    —      —      78    102    —      —      102