I have a Full-Size spare wheel with an unused Conti Tyre - manufactured in March 2015. Rather than buy 4 new Conti tyres, what do you think about buying 3 and using the spare?

From Continental website- 10 years

Continental is unaware of any technical data that supports a specific tire age for removal from service. However, as with other members of the tire and automotive industries, Continental recommends that all tires (including spare tires) that were manufactured more than ten (10) years previous be removed from service and be replaced with new tires, even when tires appear to be usable from their external appearance. Even if the tread depth may not have reached the minimum wear-out depth. Vehicle manufacturers may recommend a different chronological age at which a tire should be replaced based on their understanding of the specific vehicle application; Continental recommends that any such instruction be followed. Consumers should note that most tires would have to be removed for tread wear-out or other causes before any proscribed removal period. A stated removal period in no way reduces the consumer’s responsibility to replace tires as needed.

Your car will not be worth any more with a new spare than a used one. Go ahead and use it. It will be fine!

Of course, it is.  SVR 2018 in Santorini Black.

If you are old enough, in the old days we used to rotate wheels every 10000 miles or so to have even wear all round and even LR up to recently did it for you at a service if you car came with a full size spare. With the advent of space savers or even just a can of foam this is something that has fallen by the wayside. 2020 Pangea Green 1st Edition D240 New Defender 110 is here and loving it
2018 Melting Silver Mini Countryman PHEV - soon to be replaced
2015MY Corris Grey SDv6 HSE Dynamic, the best car I have ever owned, totally reliable only a cou0le of rattles in 3 years, now no longer in my care
Also in my garage is a 1996 TDi300 Defender 90 County HT made into a fake CSW

Great! Thanks for your feedback - that will save the cost of 1 tyre.

What do you think about the Continental Cross Contact LX Sport tyres?

I am sure there is nothing wrong with them at all. A lot is down to personal preference. If you want to keep a nice set of 4 tyres the same, then the choice is obvious.

I tend to go for less expensive brands these days. All the cheaper brands are owned by the premium brands and have the same quality controls. I will be needing 4 new ones on my 2014 soon, unless I sell it. On my last car I had yokohama which were superb, but did not last long, Nexen which were great, and Barum which handled badly. I would avoid those at all costs.