Went back to week 6 run 3 because I've been failing at week 7. (Of course, then I realised they're the same - 25 minutes.)
But it was all a bit irrelevant because I only managed 12 minutes before having to stop. My calf muscles felt like concrete instead of stretchy elastic. Dammit! I feel like I'm never going to get past week 6/7 and get to the end.
Have you tried a foam roller? I started using one for knee pain a few weeks ago and after just a few days noticed that my quads were very much softer and the pain was less. It's not exactly a fun thing to use but it worked for me. 
It could also be that you're getting fitter and takes you slightly longer to warm up. When I got to week 7, it took me about 10 minutes before my legs wanted to move, but after that time I usually got into a good rythym. I still find that the first 10 minutes are where my legs feel tightest. I find the same on my bike; i'm used to cycling 30 miles at a time, so it can sometimes take me at least 5 miles before my legs actually get into a good rythym.
Another thing I'd suggest is a cool bath. Don't go for ice-cool; just colder than what you're used to. There are different ideas on how it's supposed to work, but I found that it released all my stiffness during the first few days of 5x50.
Another thought - have you tried going a bit slower? Maybe you're so desperate to do it, that you start off too fast? I did that this morning, and a bit of a hill didn't help either. Once I slowed down again (and was off the slope) it all got much better.
I have suffered with pulls and niggles in my calves throughout the programme. I would definitely suggest getting a foam roller - it has worked wonders on my tight muscles. As Greenleg suggests, watch your pace - also try and lean forward a little when running (not at the waist, but just shift your centre of gravity forward slightly) also take light, but small (not bouncy) steps landing on the mid to forefoot rather than the heel. Finally, stretch really thoroughly after running and, if you get one, use the foam roller daily. It has taken me 18 weeks to develop a running style which minimises the stress on my joints and muscles and, at the risk of tempting fate, I will finally be graduating this week
I remember my legs feeling so heavy, like lead in those weeks! I think it is because you are doing the longer runs and your body isn't use to it. Try to slow down as suggested. Looking back, we would of benefitted from taking an extra recovery day between runs. Gayle
If you haven't given up altogether, you haven't failed. You are a work in progress, building up a head of steam.
Try going considerably slower....you will just be re-distributing your energy in a different way and it may surprise you how much further you can run.
These words are sent to you today from a very very slow joggy-walker. Slow, but happy!
PS. Haven't you just recovered from flu?
Thank you all for your encouragement and tips. I won't give up!
I'll try your suggestions and let you know how it goes...
Failed, Failed, Failed
Failing to plan, plan to fail.🤔
Fail!
Failed
Fail!
