46/30 crankset on a Giant Defy 4?

For a triple you would need a new front derailleur as well as chainset and STI.

If you put a 32t cassette on the back (your rear derailleur may be fine, otherwise slap on a 'GS' medium cage one) for 28" lowest ratio, which is pretty low with 34t chainring. Dropping a 4 teeth on the front won't make a huge difference, bottom ratio on 30x32 would be 25".

You won't get much lower gearing without a MTB style chainset, which probably requires a lower clamping point than on your Defy.

I believe you can run 36t cassette with a MTB rear mech (the 9-speed XT?) but would have to dig through back issues of CTC mag for the details.

Don't forget to work on your climbing technique - pedalling, breathing, posture. This will be of far greater benefit than any amount of gearing, lightweight stuff, carbon, drillium etc. GCN videos are good and most road-oriented sites discuss climbing technique regularly. Like anything worth achieving, it takes time and effort, but is very rewarding.

Edit: just seen your reply to bushman. Hope you get something sorted.

Fellow Northumbrian from Cycle Art here. We do this kind of conversion all the time so come and see us if you like. It does make more difference changing the sprockets at the back but if you prefer the chainset route we can do that to. You may have chainline issues trying to use an MTB chainset but Im sure we can work something out. What size cassette do you currently have?

Depending on what model fsa crank you have you could just buy new chainrings in those sizes (or as near as you can)

http://www.chainreactioncycles.com/chainrings?f=2259,4294966649

Or buy a triple crank, most have 50.39.30 chainrings

http://www.chainreactioncycles.com/cranksets/road-cranksets?f=2259

Fellow Northumbrian and anti gear snob here too.

The cassette and mech option (you'll also need a new chain) is probably the easiest option but if claris is 8 speed you are limited to 11-32 at least as far as I can see. There are plenty of options for the mech and a long cage 8-9 speed mtb is probably your best bet.

If you want to go the chainset route then you could get a road triple and run it as a double with the inner 30 ring and an after market 46 in the middle position using single chainring bolts. You might need a different front mech but I suspect it will probably work OK.

Option B for chainsets is to fit an MTB double but you would likely need spacers at the bottom bracket to correct for the axle width.

Good luck

There's no shame in running a smaller crankset and Shimano do a CX50, which is designed mainly for cyclocross set-ups, but in principle there's no reason why it shouldn't work for you. The black-magic will involve checking whether it's an easy swap with the existing bottom-bracket, which again in principle it should be as they're both Shimano, but I'd suggest getting help from a local bike shop if you're unsure.
The other option might be to whack a bigger (dare I say it, mountain bike) cassette on the back, but that may need a longer-cage derailleur as well and unfortunately Shimano mountain bike derailleurs aren't as compatible as they're road brethren, so agin the LBS might be needed.
Just for the record, I'm running two cyclocross bikes - a Kinesis with a standard compact chain set, it's fast, but can be hard to ride in the steep rough stuff - and a Cannondale CAADX with a 46/36 chainset that's clearly happier on harder terrain, but has equally done sterling service piling on the road miles during the winter, so a cyclocross chain set isn't as daft as some might think.