So I thought I'd see if I can put that little canard to bed using everybody's favourite bedtime reading - the UK Blue Book 2011.
MMT uses a sectoral balance approach based on the now familiar sectoral equation.
 (S - I) + (T - G) + (M - X) = 0and this is derived from two GDP equations:
 Y = C + G+ I + (X - M)and
 Y = C + S + TThe UK Blue book is based on the ESA95 derivative of the system of national accounts, and describes only equation  under the 'Expenditure Approach' in Chapter 1.
Where C = final consumption expenditure by households and NPISH sectors, G = government consumption expenditure, I = investment or gross capital formation, X = exports and M = importsThis is then derived in table 1.2
The values of (S -I), (T - G) and (M - X) are the sectoral net lending figures from the financial account and are showing in table 1.7D under point B.9. Putting those into equation  gives you the following
 (117133) + (-150151) + (33018) = 0In dataset CDID terms the equation is as follows:
(EABO+NHCQ+NSSZ+RVFE) + (NNBK) + (NHRB)where RVFE is the statistical discrepancy.
So the rest of this post is about how to derive those numbers from the UK Blue Book accounts for the year 2010.
First lets put the standard numbers into equation  from table 1.2
1458452 = 937906 + 338067 + 224577 + (436796 - 476480)That is out by 2414 which is the statistical error of the expenditure calculation over the income calculation.
Now we need to slightly adjust this equation. The value for 'I' includes government expenditure on gross capital formation (GCF). So I'm going to move that to 'G' where it belongs in the sectoral view. So we look up the Government GCF which is P.5 in table 5.1.7 and see that it is the princely sum of 36488. Substituting that in gives:
 1458452 = 937906 + 374555 + 188089 + (436796 - 476480) - 2414In dataset CDID terms that is:
YBHA = (NSSG) + (NMRK + NNBI) + (NQFM - NNBI) + (KTMW - KTMX) - RVFDEquation  isn't one of the standard GDP measures in the text, but we can find the elements. We already have 'C'. 'S' is 'UK Gross Saving' with the government sector removed: B.8g from table 1.7D. T is 'General Government Gross Disposable Income' from table 5.1.5. Substituting that into equation  gives us:
1458452 = 937906 + (185437 + 98397) + 239670which is out by 2958 - and that happens to be the difference between the GDP figure and what is called Gross National Disposable Income (B.6*g in table 1.1). So we have:
 1458452 = 937906 + 283834 + 239670 - 2958In dataset CDID terms that is:
YBHA = NSSG + (NQET - NNAU) + NNAO - dGNDIand
dGNDI = YBGG - QZOZ - IBJL - YBGF - QZEPNow we combine equation  and  and shuffle it around into the format for equation 
 (283834 + 2414 - 188089) + (239670 - 374555) + (476480 - 436796 - 2958)(S - I) gets the statistical discrepancy since it has that in equation . dGNDI is to do with foreign income outside the definition of GDP and is assigned to the external sector.
Collapsing  down gives us the raw sectoral balances:
 (98159) + (- 134885) + (36726) = 0Now we need to adjust for the capital transfers at D.9 of table 1.7D and acquisitions less disposals of non-produced non-financial assets at K.2 of table 1.7D, which gives
(98159 + 23806 - 4010 -822) + (-134885 + 16152 - 32297 +879) + (36726 + 1077 -4728 -57) = 0and simplifying that down gives us equation  again as expected:
 (117133) + (-150151) + (33018) = 0So the answer to the equation I + (S - I) = S is:
188089 + 117133 = 305222Not quite as succinct as '42' I admit, but of similar portent.