The larvae of the Hemerocallis Gall Midge (Contarinia quinquenotata) are serious pests of Daylilies (Hemerocallis sp.). It infests the flower buds causing the flowers to remain closed and rot.  There are many white larva inside between the thickened and distorted petals. 

https://www.rhs.org.uk/biodiversity/hemerocallis-gall-midge

Gall Adult

Provide a photo

In gardens and parks, where the host species of Daylily occur.

When Daylilies are in bloom.

Widespread in Britain, though less frequent in Scotland.

The Royal Horticultural Society is tracking its spread through the UK, and have asked for records to be submitted via their hemerocallis gall midge survey 

Appears to be frequent in Leicestershire and Rutland (which are mainly from records on the NBN from the RHS survey)