I have got it nearly synced now, but there is clearly something fundamentally wrong.

The wallet will sync for a maximum of three minutes, after which all incoming network traffic appears to stop but some outgoing network traffic still occurs. This is very repeatable, except that sometimes when the wallet is started there is no network traffic in either direction. There is nothing wrong with the pc - that runs other wallets (and miners) without a single hiccup.

I have seen/had problems myself with DNS and wondered if this might be a factor? Are the seed nodes hardcoded into the wallet as domain names or IP addresses?

Edit: The wallet synced and had been idle for some time when it just crashed with an assertion failure