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Hi, took above vehicle to local dealership for service, replace radiator and replace rear door handle. Three days later car returned. Following morning, battery flat. Absolutely no problems with battery prior to this service. Return vehicle to dealership. They say nothing wrong with battery - charge battery and return. That evening battery flat again within two hours. Next day return vehicle to dealership. They kept vehicle two further days testing it to see what was draining battery. They were adamant nothing wrong with battery itself. Eventually on the 5th day vehicle returned with new battery and invoice to match ($1836.65) claiming the battery was at fault. Throughout it had been explained that similar situation occurred approx three years before this and that they should take it to specialist automotive electrician. On that previous occasion Vehicle taken to specialist auto electrician by this dealership, who located minor drain on battery and made repair relatively inexpensively. Our concern is that in doing the work asked of them in this last service etc, they've done something to cause the battery to drain, eventually after 5 days, located the drain, repaired the problem they caused and replaced the battery (that was good when it went in there) and charged the hours for labour to recover their costs. Does all this sound reasonable on their part or are we being unjustifiably suspicious? and is $1836.65 reasonable in this situation?

While it's impossible to say for sure, I don't think the repairer has caused the problem. Occasionally, a battery can develop a poor internal connection and can test perfectly, only to fail again shortly afterwards.
It is possible that if the battery was weak, it was flattenned while they were working on the vehicle (perhaps by having the door open and interior light on??). However, there's really nothing they could have done to cause the battery to fail and it's unlikely they introduced a drain by repairing the radiator and door handle.
On the balance of probability, I think they are telling you the truth and the battery seemed fine when first charged and tested, but it was in fact faulty.