Intermittent boot lock won't open fault on 190E - Appreciate help please?

I have a fault which generally only manifests itself when I go to open the boot for a second time, after switching off the engine. IE, switch off car, walk round to boot, opens fine,
shuts, gets indoors, realise you need to go into the boot again, button goes in, nothing else happens. Put key in lock, activate central locking by locking then unlocking it, boot opens no problem.
I presume from what I read on here that some sort of vacuum leak is likely, but I'd like to know ball-park figures for the repair, or if it's worth bothering with.
Many thanks for any useful comments.
-- Dave; Who thinks usenet is now getting quite short tempered enough, without me joining in too!

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