Thanks for your thoughts and support, chaps.
The question of whether to use a kit or go on a voyage of discovery (well, it would have been for me) and scratch build is a moot point. In this case the basic dimensions appear to be correct but it is possible that the firebox may be a bit short in height. Bear in mind, though, that the photos above were of each of the parts rested in position and it looks as though the front of the firebox may have sunk a bit.
Overnight I've come up with some thoughts which I'll share below. However I agree, Fraser, that the absence of any front to the firebox is a glaring error, and the equal absence of any drawings or details about how to form the front within the instructions is an equal mistake. I can probably bodge a front and may well try that before joining boiler and smokebox. Neither is there any fixing provided for firebox to footplate, although if the boiler and firebox are joined and the firebox is then soldered to the cab front I guess the fixing would be OK. That is not, however, part of my GRAND PLAN. In fact I've borrowed the book of cunning plans from a certain other thread.
Boiler dimensions look pretty close to the GA, Simon.
My current thoughts are these:
1. Make a firebox front which needs to only approximately fill the hole and solder in position.
2. Use the second spacer to make a firebox back and solder this in position too.
3. Shape the firebox front using "Plastic Metal" which will withstand soft soldering temperatures (which I know from practical experience with the tender). It will also avoid the embarrassment of a soft solder filling melting as I get a soldering iron anywhere near the beautifully formed firebox front after it's been filed to shape. Shades of experience with the tender there as well.
4. Join boiler and firebox. This may involve a bit of nifty work with introducing some studs which can come from the boiler in to the firebox in order to locate it properly as there'll be b****r all brass on the front of the firebox to solder to. Hopefully the location, height and parallelism of the boiler/footplate can then be checked and adjusted as necessary. New boiler mountings might well be needed anyway so this will give chapter and verse on that aspect.
5. Fill the space in the cab front (I'll post a picture shortly to show why this will be necessary), or make a new cab front.
6. Using the location tab provided on the firebox and the slot in the cab front tack solder one to t'other and drill through the cab front and firebox back to provide holes for BA locating screws with the nuts soldered to the inside of the firebox back. This will, with a bit of nifty footwork at the front end allow the boiler to be removed for painting etc.
One of the joys I have yet to deal with - and it'll come a lot later - is that the provided backhead is etched. I'll just stop a moment to let that sink in..... In due course I'll post some photos of that, but if I can bend it to shape, fill and file properly there should be adequate space behind it to hide the screw securing the boiler/firebox to the cab front. (Regrettably, despite searching for several years, I've not found a suitable cast backhead).
Dunno whether I'll get out to the workshop this afternoon but I'll try.
Brian
PS - Having just read Roger's update - oh dear - I didn't check the castings for a front. I sorta assumed that, as there are bend marks and half etch marks included on the firebox etch that's what is provided. I may yet have to eat my words.......