How to gain admissions to graduate school with postgrad diploma, but no degree? At the age of 21 I entered an (UK) M.Sc./Postgraduate Diploma Programme in Software Engineering.

I entered without an Undergraduate degree based upon modules I undertook in a distance learning Masters Programme from a reputable UK University that required no formal entrance requirements (very unusual situation that I took advantage of).

Out of a cohort of 40 students on the new M.Sc. course I was the only one without an Undergraduate degree although I had Postgraduate credits as mentioned. Again this was unusual.

I graduated with a UK Postgraduate Diploma (but not an M.Sc. as I could not fund the remaining twelve week/3 month project), however my marks were excellent.

Here I am twenty years later with no degree. I wish to undertake further study in Software Engineering and am at a loss to effectively know where to start. I am way too late to go back and do the three month research project to get my M.Sc.

[For those outside the UK a Postgraduate Diploma is effectively the taught/exam component of an M.S/M.Sc. and usually compromises 75% of the course. It is an M.Sc. in all but the three month research project. Most students complete the project but some do not, usually due to funding or academic reasons. My reason was purely funding.] 

I am not qualified for an Undergraduate Degree in Software Engineering (effectively a waste of time as I'm somewhat beyond that syllabus) as I do not hold School level University entrance qualifications and I'm not qualified for an M.S/M.Sc. ironically (even though I previously were on an M.Sc!) as I have no first/Undergraduate degree.

If I had finished the extra 12 weeks I'd be qualified to enter a Ph.D. programme but the failure to do the twelve weeks (wholly due to the fees at the time which were huge for the Project) makes me qualified for nothing even though I hold a formal Postgraduate level qualification.

What to do?
I can't vouch for the UK, but here on the other side of the puddle, once you've been out in the Real World (TM) for some number of years, your classes start mattering less and what you've done with them starts mattering more. A school's concern, if you're coming back for an advanced degree, is whether you have enough knowledge to cover the prerequisites and enough aptitude to complete the degree in a reasonable amount of time with a reasonable grade. (In other words, whether you'll be a student whom they're good for and who makes them look good.)

For PhD there may be some bias toward folks who are already researchers, but "research" comes in many flavors, not all academic.

(I could have gotten into Columbia in NY for a MS simply on the basis of "you can't be an idiot or IBM wouldn't have hired you" -- plus having aced a few of their extension classes. Can't imagine things are very different there.)