Thanks for the excellent question.
Firstly, do you actually need to do research? If you want to work in a competitive speciality (especially in a tertiary centre) and in certain locations (I guess London would fall into that category), then yes I would encourage you to do research. Consultant job descriptions will have ‘expected’ and ‘desirable’ characteristics and some research experience or a higher degree of some description will be in there. However, it is not required unless you are taking up an academic role in which case it is an absolute must.
Research can come in several varieties - clinical, laboratory based, drug trials etc - but to be honest, the principles you gain from doing research is really more important than the actual thing you are researching. It is a distinctly different pace - some find it boring, some find it relaxing - but it is a nice change from clinical life. I took my research time in 2015 - 2017, after 9 solid years of clinical practice, exams and on-calls - it is a nice ‘career break’.
Research degrees come in MRes (one year), MD (two year) or PhD (three year) varieties and can also be done part-time doubling their length. I would suggest an MD at least - these can be extended if you want to.
Timing - I would suggest trying to do research after your CMT as you should focus on getting your membership as the first priority and during the early part of your speciality training. This has the advantage of giving you time to get your clinical work up to par (or your procedural skills in a more ‘interventional’ speciality) and gives you time to write your thesis up.
I did my research in my ST6 year and am will be in the write up phase for several years to come. However, my research did include clinical commitments and angioplasty work so I maintained my skills sufficiently that when I returned to full time clinical work, it wasn’t a major adjustment.
Some do it earlier and it would certainly help score points on the registrar application form but I personally feel your priorities should be as above (score points through teaching, audit, presentations, publications etc - the research can wait)
A final piece of advice - you can find lots of consultants and professors with research interests who would love to have a research fellow. However, make sure you are getting something out of the project. Working for 2 years on a 10 year project that will never produce results in time for your MD completion date is not worth your time. Working on multi-centre RCTs is also great experience but again gives you no CV material (I am in the small small print of several but it counts for nothing). Make sure your research project fits your timetable and your goals and use your spare time wisely to write up other articles, side projects etc to boost your CV - again, you will never have this much spare time again!!
Hope this helps and good luck.