after a lot of tinkering this is my method of tikka'd lamb its a bit long winded but it makes dangerous tasty tikka
add the cubed lamb to a pot of sliced onions, G+G paste, water, turmeric, garam massala, tom puree, seal the lid tight so no vapor escapes [amounts vary depending how much lamb there is]
pre-heat oven on 150C and cook for 4 hours, allow to cool then marinade in tikka overnight
then a quick burst in tandoori/grill/BBQ.