Howdy Dave,
Well, this is a problem that alot more people have than we realize. I have, as recently as yesterday considered starting a thread in which forum members could scan and upload instructions AND decal sheets to a thread in the forum. However, not being sure of copyright laws in the UK, as well as not having cleared the idea with John, the founder of the forum, I have not acted on the idea.
I do however think that such a repository of information (instruction sheets and decals) would be of great assistance to modelers.
But as for your current problem.....I did abit of research and went to the tamiya homepage and in their frequently asked questions section, I found the following:
I lost the instructions. Can you send it to me? 
We recommend you contact your local retailer or local
agent listed on our web site.
So, I took the liberty to find your local agent ( I hope you live in the UK lol). and here is that contact information:
U.K.THE HOBBY COMPANY LIMITED,phone: 01908-209-480
Now on the off chance that you are somewhere else, the contact info for other countries in europe appears on this link:
http://www.tamiya.com/europe/euro.htm
Now, if you can be more specific as to what models you have, ie.... the stock number found on the box, I might can locate you some downloadable instructions online......MAYBE, but no guarentees. But I would be more than happy to look for you.
So, if you can provide the stock #, scale, and model name I will be happy to see what I can find for you.
Otherwise, you can contact the distributor in your area and get the instructions from them....I am sure there will be some sort of charge as well as postage.
Now onto the airfix sheets, try this link:
http://pws.prserv.net/gbinet.dbjames/sheets.htm this man offers instructions . BUT READ THE PAGE FIRST, it is full of helpful info.
And last but not least, Italeri. This link will take you to Italeris "contact page" wherein you can contact the manufacturer directly. You will need to be as specific as you can about the instructions you are looking for.
http://www.italeri.com/comunica.aspx
I had started a thread a few weeks ago in which I listed the contact info for several manufacturers, but it seems to have died lol, although its still here in the forum somewhere.
I personally think that info like "instruction sheets", Decal sheets and contact info for the various companies would be a great asset to the members of the forum. I know where alot of the information can be found, but the compilation of the info is not that easy. I may, in the near future put it all on my geocities website (well as much as I can) and provide the link to the forum for refference. My only problem is, after being off work for almost four months with a neck injury, I am due to start back to work on Monday Sept. 11, so my time online will be cut way down once more.
But if I can help you any further, let me know and I will be happy to do what I can for you.
have a good day,
Greg