Lavac Installation Pongy - Any ideas?

UPDATE:

Result! Having replaced inlet and outlet hoses; flushed with vinegar; installed genuine Lavac siphon break vents (no suitable Biro tops to hand, Lavac spares only £2.55, but chandler charged £5 for posting these two tiny things!); turns out the smell was.......
frying battery!

Battery compartment is immediately behind heads and internal short in engine battery was causing it to overheat. (See http://www.ybw.com/forums/showflat.php/Cat/0/Number/1865036/an/0/page/11#1865036 for full battery story)
 
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