renault kangoo warning light pre heat glow plug?

Hi All can anyone answer this for me just bought a kangoo 15dci soon after the purchase have started to have an engine management problem when the engine is put under pressure the glow plug light comes on and the vehicle looses power take your foot off the gas and the light goes out and normal performance returnes, the problem is getting worse and more frequent,, is it a relay problem etc and if so what component needs replacing ,and can you supply a brief description of the item and its location many many thanks if you reply peter