My chain is dropping off the big ring a fair bit which doesn’t help keep control – especially if you’re setting up for a tabletop! I’m riding a lot of rocky stuff which I think throws the chain around a lot more.
I’ve already adjusted the front derailleur to make sure it’s coming far enough over.
So is moving to a double and bash a good solution – with or without some kind of retention device? I ride all sorts, Peaks, Lakes, Lee Quarry so do tend to use my current granny gear but rarely use my top gear
I have a 32-11 9sp cassette and standard 22,32,44 rings.
What do I need to change, and how can I do it cheaply?

Take big ring off. put bash ring on, adjust mech stops to suit, shorten chain to suit

Buy an el cheapo bash ring like THIS ONE and shorten your chain to suit.

Ideally you should fit a shorter rear mech, but they’re pricey if you’re doing it on a budget

36 ring and bash ring or just a bash ring. CRC do a FSA clear one that’s pretty and about £15 or bargain below.
http://cgi.ebay.co.uk/Chainring-Bash-Guard-32t-36t-/150594469504?pt=UK_sportsleisure_cycling_bikeparts_SR&hash=item2310213e80#ht_500wt_1156

I changed to this setup on my SX

http://www.chainreactioncycles.com/Models.aspx?ModelID=5357

http://www.chainreactioncycles.com/Models.aspx?ModelID=17282

http://www.chainreactioncycles.com/Models.aspx?ModelID=28729 in a 36T

I haven’t used it loads but so far no problems , I might get round to changing the mech to a shorter cage version but it’s working ok as it is at the moment.

you will probs need some longer chain ring bolts. And you will need a dual chain device like the one superstar sell for £16. As said shorten chain up, and bobs yer aunties live in lover.

You don’t need a chain device and the ebay bash has the chainring bolts with it for £7. Just buy it now and try it for that money. As long as you have four bolt 104 bcd cranks.

Like the other bloke said – old ring off, stick one of these on – http://www.bbgbashguard.com, adjust top limiter and remove some chain links if you wish.

Is a bash ring really necessary? With the increased clearance from dropping the 42t outer ring, does anyone regularly ground the middle ring?

Or is it to protect from loose rocks, and help keep the chain on?

I ask because I’ve just set up a 1×9 hardtail and that bash on ebay looks a bargain but do I really need it??

A bash ring will protect you from your middle ring….

1×9, probably not, assuming you’re some chain retention device. generally speaking, ditch the big ring, shorten the chain, you’re good to go. long chain to run big ring means dropped chain. shorter chain, dropped bigring, means less chain drop.
either get shorter chainring bolts ( hubjub or charliethebikemonger ) to run the middle alone, or get a bashring.
and yes, i bash my bashring a lot. not just for fun. 😀

If you don’t like the look of any of the above mentioned bashguards then theres a chance you might find the shimano hone/saint one acceptable. Swinnertons have the 32T one for £10 (listed in spares somewhere I think) No bolts though unlike the big-enough-for-34T-but-named-32T-clear-FSA-one

You will also ned to purchase some longer bolts for the bash, CRC do them.