Multivision no longer exist in terms of your warranty.Some of the assets of the ex Multivision company have now been purchased by other companies, one being Carrera. Carrera are offering to continue the warranty on a yearly basis for about £90 approx.Rock Direct are also offering this warranty extention for the same price.
As you purchased by credit card, then your credit card company will be responsible for the remaining warranty, under the Consumer Credit Act. Whether they undertake the arrangement for repairs, or contract elsewhere, then it would be your responsibility to find out. Regarding adding on items should not invalidate warranty if common sense prevails, but of course, this would depend on how far you have gone with this.Major upgrades from the original specifications of the machine, would perhaps cause some concern, and the credit card company may refuse to cover your upgrades. Again, it would be your responsibility to contact your credit card company, so that you can clarify the situation.I would do this now,so that you have knowledge of your credit card company's stance on this issue, and not perhaps at a later stage.