Having spent two weeks wrestling with Easily.co.uk (a client’s chosen hosting company) trying to get the Microblog stack from Stacks4Stacks to work - as it has elsewhere without problems - I eventually convinced the client that the best course of action was to move hosts.
I chose the Rapidweaver-friendly Chillidog Hosting, run by @barchard. It took five minutes to create a new hosting account, another 30 mins to change some behind-the-scenes stuff, and then a minute to upload the site.
Microblog stack works perfectly first time (as it has everywhere else).
The moral of the story is obvious: the cheapest option isn’t always the best option.