Yep. Bollox aint it. The garage a few doors up asked my if i was going to have my P100 IVA'd because ive changed it from leaf to IRS and its now sat on cossie running gear. But of course i haven't done that and i have photoshoped all of the pics in my build thread to make it look like i have

Read the DVLA website, its all to do with how many " major components " you change.

2. Vehicles that have been
rebuilt using a mix of new
and/or used parts
To keep the original registration number:
Cars and light vans must use:
n the original unaltered chassis or unaltered monocoque
bodyshell (that is, the body and chassis as one unit);
or
n a new chassis or monocoque bodyshell of the same
specification as the original. A receipt from the dealer
or manufacturer is required.
AND the vehicle must have two other major components
– as listed below – from the original vehicle.
n Suspension (front and back)
n Axles (both)
n Transmission
n Steering assembly
n Engine
If a second-hand chassis or monocoque bodyshell is
used, the vehicle must have Individual Vehicle Approval
(IVA) or Single Vehicle Approval (SVA) (see section 11). A
‘Q’ registration number will then be issued (see section 7).