Ignition or battery warning light goes on & off at will on my 07 TDV8 whilt driving. garage has diagnosed replacing alternator. Seems like an expensive stab in the dark to me, anyone else had this problem?

Sounds about right to me. Expect the alternator to fail fairly soon. Mine lasted less than 75 miles after it started to show an error.

It might be worth checking electrical connections too, of course, in case it's just a loose connection somewhere although the alternator isn't that easy to get to. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

Almost certainly the Alternator - mine had exactly the same symptoms.

One of the most common failures on a TDV8 apparently. Cost to replace at my local indie was around £380 and they did say afterwards it was a pig of a job to do.