For the most part the interim service comprises extra items on top of the engine bay service in that work involved is on the underside of the vehicle, normally brakes, steering and suspension.
The main reason for not including the above areas in the engine bay service is that these other car parts and components have, by and large, a longer effective working life and are far less susceptible to accelerated wear due to the shorter and slower school and shopping runs that the engine has to contend with.
As with any maintenance procedure the keyword is prevention, far better to strip the brakes and find a leaking brake cylinder requiring replacement than ignoring service intervals allowing the cylinder to leak brake fluid all over the brake shoes necessitating their replacement as well.
The checks on suspension would normally start with a bounce test on each corner of the vehicle ensuring that after weight is removed the vehicle rises through its normal ride height, drops back through the normal ride height and finally rises for the last time to the normal position. Should the vehicle rise and fall more than 2 or 3 times then the shock absorbers (dampers) will require further investigation. Excessive bounce is normally caused by a defective valve within the shock absorber allowing little or no restriction to the passing of fluid between the upper and lower chambers or failure of the top seal, allowing leakage of damper fluid to the outside. In the first scenario no external leakage can be seen but in the second fluid will be easy to see around the outside of the damper body and in extreme cases even dripping from other suspension/steering components. Replacement of the defective unit must always be done in pairs i.e. right hand front failed replace both front, the same applies to the rear, this replacement in pairs ensures that the damping effort is even on both sides preserving the handling and safety of the vehicle.
Moving on to the other main suspension components we come to the road springs or their torsion bar and fluid equivalents. In the vast majority of cases any faults here will be clearly evident as a broken part or as a fluid leak, a good powerful torch or lead light is vital as some coil springs have a tendency to snap high up in the inner wing and if only a six inch section has snapped off little difference will be made to the vehicle ride height. Fractures to a torsion bar will normally cause the vehicle to drop at one corner, not exactly difficult to spot! Much the same applies to fluid suspension in that the vehicle will lose its normal ride height at one corner, or more commonly, all of one side or all front all rear.
Other important suspension and steering parts are the various ball joints and linkages and a good look around will usually identify these components. The assessment of wear normally takes the form of raising each wheel and shaking it firmly with the hands placed at the top and bottom of the tyre and then at either side. Any free play may need to be identified by an assistant looking behind the wheel area. Excessive play needs to be addressed as the adverse effect on steering alignment will almost certainly affect the life of the tyres.
Whilst under the vehicle have a good look along the full length of brake and fuel pipes and hoses, any damp areas must be investigated and rectified, particularly in Vauxhall parts, check also the security of these lines ensuring that they are clipped firmly to the floor pan. On the subject of brake and http://www.carsparefinder.com/features/how-to-replace-fuel-lines-in-your-vehicle/ can only be carried out with the engine running and the brakes applied.
One final word, although this should really be the first, is that of safety. Work on the underside or indeed any procedure involving raising the vehicle should be done on a hard concrete surface using a trolley type jack operating well within its lift capacity and once raised the vehicle must be supported by correctly positioned axle stands.