It's certainly an area where you can save yourself a fair amount of money. Let's face it we all trust garage staff to do everything on the service schedule, but in reality car servicing across the nation is largely a rip off. Most of the checks required on a service are duplicated by the MOT hence the £99 service and MOT that is so common now. So you can do the filters, plugs and oil change after an MOT and deal with the advisories too. The only thing that you can't do is a diagnostic check, but if no faults are indicated then surely that can be left until an insurmountable fault arises. I'd say go for it and mark up your own book and keep your oil and parts receipts with the rest of the history.
Car servicing has to be a bit easier now. When I did my own servicing I remember one of my cars had eighteen grease nipples on the front suspension. Brake drums had to come off all four corners to check linings and cylinders for leaks. Ignition points replaced, timing set up, tappets adjusted, back axles and propshafts to be checked as well as gearboxes.
Yes definitely go for it
let Merc do the MOT and you might get a free health check too.