I have just completed a part-time IR with Bonus at Cranfield and would recommend them highly. When I was looking round Bonus and Stapleford were the only two in the UK (that I could find) that were offering PT IR.
I am not sure where you live, but I was travlelling from Derby where I live, or after work from Birmingham (just next to the airport). It was about an hour to get there which was a big drain after a hard days work but Bonus instructors are happy to do sim work (and ofter flying) in the evening. Simming in their FNPT1 can go on to the early hours!!
I did talk to HGFC because I did my PPL there a while back and have heard excellent things about their commercial 'wing', but they didn't offer part-time at the time. Things may have changed.
One thing that you may not have considered - beware that the course will be harder and more expensive..........only joking!
As it turns out, I managed the IR in 50.4 hours so it can be done at least as cheaply (albit harder......sorry - had to get that in there :-) as full-time.
The 'trick' to a cheap IR seems to be a first time pass! As soon as you partial you will be looking at 2000 pounds+ on top of what you already paid. I confess to pulling a couple of well timed 'sickies' towards the end of my IR training on the build-up to the big day which paid off in the end.