Here is the solo that Nigel does and I'm not really sure what song it from.
Its a Bach or Mozart thing.
[ Tab from: https://www.guitartabs.cc/tabs/s/spinal_tap/nigels_tab.html ]
e|5-4-5-7-5---0-0----------0---------------5-2-0----5-2-0---|
B|-----------2---3-3--3-2-3-3----3--3--2--2------4-4-----4-4|
G|----------2------------------1----------------------------|
D|----------------------------2-----------------------------|
A|----------------------------------------------------------|
E|----------------------------------------------------------|
e|5-2-4-0-5-2-H4-0-0|
B|-------2----------|
G|------------------|
D|------------------| H=Hammer on and off
A|------------------|
E|------------------|END