I have the same problem. I'm assured that an inlet valve maintenance kit (a new diaphragm) solves the problem. Grohe part no: 43.722. Available from several places - I used Showershack, 0845.1264141. Price £7.17 plus VAT; expensive little diaphragm but cheaper than the whole valve which was about £25. I shall attempt my repair tomorrow when it arrives!
Hencot
August 2009