Boothy

I have a Jan 2000 S80 SE which I have put in for a 100k service with a local former Volvo mechanic owned garage. I have had to have the cam replaced (recommended at 108k miles) as part of the usual service. In addition I damageed my reverse gear fate/pulleys when someone had a blowout in front of me just as I was changing from 5th to 4th (or in this case slammed the reverse gate). The car was working fine in all other respects. The garage could not fully finish the job by Saturday so they kept it over the weekend. On Monday they phoned to say that they had some problems - apparently the battery had gone flat (never had any problems previously) and there were now NP systems working on the dash (petrol gauge, milegage, speedo etc). They sent it to specialist auto elec who traced the problem to what I believe is failed servo of some sort. This is the info they have given me, together with a (additional) price tage of c£900to pay. I have challenged the garage to say that these were all in full working order when I left it on Friday morning and I now have to try and find around £1,600. (no price given to me for the spark). Volvo dealer says "this is not an uncommon problem". I cannot believe that an S80 cannot survive without a battery for a short while. I cannot even drive away since it would be on the road illegally. Have I just been unlucky?? Can I get a used servo (if it is that)?