deep deep joy!

Thank you everybody who responded on my blog and the many forums that I cried for help on! I have done much researching of substituting and the muddy fog has cleared to a bright sunshine day of understanding! This is what I have learned:
Key
C220 = Cascade 220 / TDT = Tahki Donegal Tweed

Substituting - tips for success:
Weight - Stick to the same weight yarn [i.e. stay with aran / worsted / DK etc] for maximum ease and avoidance of extreme maths in pattern alteration.
C220 = worsted weight / TDT = worsted weight Check!

Yardage - This one foxed me the most! How could something with the same stitches per inch, same needle size and same weight have a different length!?
Explanation - different wools, cottons, blends etc are heavier than others. So althought they are both wool - they are from different animals - TDT must be from an animal with a heavier coat as there is less yardage per gram!

Different yardage to the gram means that the FO will be physically heavier or lighter - the greater the difference the greater the likelihood that the FO will not 'hang' right. I think I'll be OK though with this one.

Yardage must be equal or more, or I will run out of yarn before the end.

TDT has 183 yards / 167 metres per skein.
It calls for 7 skeins [in my size] therefore 183 yards / 167 metres multiplied by 7 equals 1281 yards / 1171.3 metres. [it's not hard maths!]

C220 has 220 yards / 200 metres per skein.
Therefore 1281 yards / 1171.3 metres divided by 220 yards / 200 metres equals 5.8. Which means I need 5.8 (or 6!) skeins plus 1 extra for luck! Check!

Fibre - If the pattern is wool, stick with wool. If it's cotton, stick with cotton. Generally anyway - it's all to do with drapes and how it hangs ....
TDT and C220 are both wool check!

Structure - make sure the ply is the same - apparently if you use single ply when the pattern is 2 ply it can make huge differences to the FO.
TDT and C220 are both 'plied' according to yarndex, which I figure means that it OK, so check!

Tension / Gauge - the final hurdle - make sure these match. As both balls state 18 stitches equal 4 inches / 10cm using 5mm needles then check!

Woo-hoo! Now .... which colour do I choose!
I can make Central Park Hoodie for £34.93 buying C220 from getknitted!

I am so happy, happy, happy!

Under the Hoodie saga continues ... all I have discovered is that for the pattern to work I must not be more than 0.5 of a stitch outside the designated tension requirements. I'm 1.5 stitches out, so will continue knitting my swatch and then wash it and block that mother up to size!!