1. How do I add weight to pushs ups? Instead, could I do dumbell chest presses, flat or incline?
Stick a plate on your back. If you have none, use a rucksack with a Dumbell. If you can't do these, then a single arm flat Dumbell press should just about suffice.
2. Inbetween gym sessions, I should do some cardio right? As I have dodgy knees swimming is fine right?
If you want to shift the process along faster, sure.
3. I understand that what you have suggested is for burning calories, but I did Wed session on wednesday and it was super quick. Is that fine?
If it was that quick, you need to use more weight. To the point where the final set of each exercise is REALLY hard.
4. Leg Press. What I was doing before was dropsets. So I would do amrap at 90kg, amrap as 70kg and amrap at 50kg. etc... I found this pretty good actually.
Can I stick with this, or do you suggest sticking to 3x8-5x8?
no. Use a consistent weight. Do 8 reps of whatever is pretty difficult for the first set rest for a minute, then go at it again. Rinse and repeat. You won't be doing much for your muscles by 'drop setting' them every time.
5. SLDL - Can I do this using barbels instead of using olymipic bars?
read your sentence and you will have your answer.
6. I guess there is no point adding dips to the routine?
you can if you want, although if you're pushing to workout as hard as you should, then this shouldn't even be a question. If you have a burning need for dips, stick them in on Monday...
---
Diet:
After dinner, I do tend to eat dried fruit (apricots, dates, figs) and maybe a few nuts, as well as a 0% fat yogurt.
Im guessing I need to stop this as well.
My preference would be to have them straight after your workout. if you can't do this because of logistical issues, then swap them for nuts. The yoghurt is a red herring, anyway as the carbs in that will do you more damage than the fat.
What about tea, with semi skimmed milk. this is fine right?
as long as you're not necking the stuff every five minutes. Twice a day won't hurt.