Okay, the Japan prices were too good to ignore for me so I've done a little research.
The parcel forwarding company I linked to above (tenso) seem to have a good reputation and have been around a good while. In fact the only negative things about I could find was a little high costs on shipping (we are talking about literally a few £'s here). So I've registered for an account with them and they have supplied me with a Japan shipping address for Amazon. Again, from what I've read, they are quite thorough on checking you are who you say before shipping to you and require id. I see this as a good thing.
They ship via a variety of methods, the best of which seems to be EMS. The estimated cost of shipping will be 580 Yen service charge + 5620 Yen Courier cost (I estimated total weight at 2KG) + 100 Yen for extra insurance (you get 20,000 yen insurance included in the courier cost then 50 Yen for each additional 20, 000 Yen of coverage. The Amazon price is 58, 126 Yen so this seems right). So total shipping cost is 6300 Yen = £34.71.
They also have a guide showing how to add this address to Amazon.co.jp.
I've also registered with Amazon in Japan. This is really easy as the site can be viewed in English like this:
Click on the link for the M3 and EVF:
http://www.amazon.co.jp/Canon-ミラーレス一眼カメラ-ブラック-EVFキット-EOSM3BK-BODYEVFK/dp/B00T90FB8I
On the right of the page, just above the social media links is a "click for English" button.
Registering with a UK address is no problem at all (and will be needed as a credit card address), I then added the Japan address as a shipping address. So far so good.
I've gone all the way through the purchasing process right up until the final click to finalise order page and all is fine.
I'm going to call Tenso on Monday, firstly to check they are easy to deal with on the phone (in English their website says) and then email them screen shots of my address's in Amazon to double check them for me. If this goes smoothly then I'll be ordering this way on Monday. I've read elsewhere that the almost free EVF offer is only for the first 25,000 units sold in Japan.
I did think about the guarantee, but I'm buying from Amazon in Japan, not some company I've never heard of. It will be a pain having to ship back again but worth the risk I think.
The M3 plus EVF would cost £800 in the UK.
From Amazon Japan with international shipping costs and insurance it will cost £355 + plus import duties and taxes of about £100 I reckon.
Quite a saving.
I've never ordered anything this way before. What do you lot think?
EDIT - I did a little more checking on Tenso and found they are widely used by Rakuten. They had a big promo with them just a few months ago:
http://global.rakuten.com/en/event/tenso/
Makes me feel more comfortable tbh.