mgaguru.com/mgtech/cooling/cool_121.htm

If the OE coolant is OAT and you've put Ethylene Glycol in, then i'd drain the lot soon as, flush it out well and replace the lot.

www.honestjohn.co.uk/forum/post/?t=19701

This is an interesting old thread which might help. You should have used the same stuff but I doubt if a few mls of Bluecol would be too deleterious. Its main ingredient is what is in your red coolant but there is apparently a risk of it becoming glutinous if certain types of other coolants are used.

With modern designs of cooling systems it's amazing how little attention they need. I have only once topped up (with only 200mls rainwater) our 16yr old Focus. Still looks clear and pink and a sample remained liquid in our freezer.

The short answer is: YES

You should drain and flush and refill with an appropriate product. While you’re at it use a suitable system cleaner product (use this with tap water). Try your local factors and get something from the Comma range. I think they do a guide on their website.

Don’t worry about it being 50/50, 25% will do fine for the UK climate, but DO buy a tub of deionised water with it.