Hi I will try and help
can you tell me exactly how the covenant is worded? i.e. can you re-type it out here
will you be separating the property physically or building another house on the land in order to let?
in relation to the covenant my feeling i that you wont be in breach of the covenant. There is case law that indicates that indicates that where a covenant is worded "private dwellinghouse" this would included multiple dwellings, however where the word "a" is also used this catagorises the covenant to only one dwelling. IN these case however someone was actually building another property on the land, which is not your intention. You are not actually creating a new property but occupying the same property but with another family. The covenant does not say that the property is only to be occupied by one family so that is the reason I think you will no be breaching the covenant.
Two other considerations for you (1) you may consider getting insurance to cover the risk in the other side taking issue and claiming a breach. Insurance for an anticipated breach can be quite expensive (and indeed may not be possible if the insurance company think it is too risky) but may be worth investigating. That way if someone tries to take issue with your activities (whether correctly or specutively) you just pass this to the insurance company
(2) the joint occupation may be in breach of your mortgage conditions, so you need to take that into account
there are quite a few out there. One that I use are http://conveyancinginsurance.co.uk/
however the polocy will have to be quite bespoke and not "off the peg". To ensure that it covers you sufficiently you may wish to employ a Solicitor to put a proposal together ensure that you are covered
Hope this has helped, if so please leave me positive feedback so I can be paid for my time. The question wont close and you can ask any further questions if you think of them at a later date.