Mark welcome to the forum

I worked on a house a few years back where they (for some obscure reason) fitted the lintels two courses above the window frame. All was well because the house had substantial wooden windows then they swapped them for UPVC and had almost exactly the problem you face. The bricks being loose above the lintel is a worry and IMHO requires investigation. although if Jerry comes along he would be better able to advise.

Fitting a new lintel is not necessarily a massive job but find a competent builder to do it. At the house I was working on the external lintel was removed and moved down 2 courses.

From a woody perspective, the hefty piece of wood screwed to the outside is a non starter not just practically but you would never be able to sell the house in the future.

Do the job properly, you won't regret it.

I don't like the sound of your temporary wooden splint, particularly as you say there is a crack up there already and loose bricks. In that case you really should consider fixing the lot while you are at it.  

Did the same builder knock through the chimney breast? If so did he remove the chimney above or support as well as he did the wall above the window?

Edit: I'm not a professional so if this is totally wrong and the house falls down don't blame me.... I suspect a professional would check the inside wall as well and do both at once. He would probably make several holes right through both walls well above any problem areas. Insert supports right through the walls and then prop the ends of these supports inside and out. The props would need to be set carefully so that they are the right number along the wall and far enough out from the wall to allow work on the wall. Space constraints may require the lintel to be placed by the wall before the props are installed. The props remain in place until the entire job is done. Only then are they removed and the small holes bricked in.  The presence of the old chimney might mean additional support is essential.

Check your house insurance covers any damage if the lot falls down. Ditto your life insurance.

[quote author=mark_k  link=1154199977/0#6 date=1154287462]I'm not affraid to try a job myself - before I attempt someting I ask for advice on how to do it (that's what I'm doing on here (this site's called 'Ask The Trades' isn't it? so that's what I'm doing) The only answer that I'm picking up at the moment is 'Get a tradesman to do the job' which isn't helping.
[/quote]

It may not be helping, but it is good advice non the less.

Why not compromise? Get a builder to move the lintel and then do all the making good yourself, that should save you about 50% of the cost.

To quote mark_k

"I know it would be better to leave the job to a professional builder, but I want to do the job myself"

There you have it chaps, he's gonna go about it anyway by the sounds of it so I don't see a problem with giving him some pointers.  
 I've never done this kind of work before so the only advice I'm going to give would be: Get a hard hat!  

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You cannot safely do this type of work with just a few pointers.  You have to know everything from start to finish and have all the necessary tools, plant and materials ready.  What if we gave him a few pointers and he attempted the job but got injured or destroyed half the house?  Would that have been good advice?

It's called 'Ask the Trades', you have asked and been advised not to do this yourself as a completely untrained and inexperienced person.

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no you dont gey the boot for having an oppinion or even disagreeing
you dont even get the boot for abuse[unless you dont heed copious warnings]

ex member is where someone removes there account

and only the posts they chose to remove are removed

if the posts went with there resignation a lot of threads wouldnt make sence

Perhaps he doesn't want any evidence that he was advised not to do it himself.

His life insurance might not pay up if the whole lot collapses on him