Further to John Lancaster's request to separate the Bachmann Deltic analysis from other issues, I can mention another area of detail that I missed out from my review (although this is getting into nit-picking territory!).
The boiler roof section (that incorporates the engine and boiler exhaust outlets) varied in detail depending upon individual locos and which period of their respective lives. Bachmann have modelled the most common variation showing the slotted boiler exhaust blanked off. On 55020 (and 55021), this slotted port was omitted altogether from new. Some Deltics (incl. 55006 and 55022) had this port removed and the hole in the roof covered with a blanking patch.
When orignally built, the first 20 production Deltics had the slotted boiler exhaust in use. Subsequently, the twin boiler exhaust arrangement was introduced, these being the 2 ports near the top of the roof and the redundant slot was blanked. Most had this conversion done in 1962. A few Deltics also had experiments carried out on them to try and overcome boiler troubles. As you might imagine, this is an area for detail that is not usually clearly seen in archive photos and film footage.
I wouldn't really expect r-t-r manufacturers to cater for the above minor modifications, but it might be useful information for those who enjoy super-detailing their models.
Ian Strange