Thanks Ian I have read that the bolts could loosen as well see:
http://forum.landrovernet.com/showthread.php/207836-TD5-Cylinder-Head-Questions
The chap also fitted steel dowels on his 10p engine and I have read that steel dowels could crack the head. I mentioned this to my garage a long time ago and the mechanic said that its not necessary to try to tighten the bolts. To me it would seem obvious that if the gasket can be compressed it could stop any leaks. When the head gasket was replaced last year under warranty perhaps they reused the head bolts and this is not recommended as they can stretch.
