206 HDi 2004 estate turns over but won't start

Hi, I'm asking for help please to try to solve a problem my step-dad has with his 206. Firstly he is a pensioner at 85 yrs old and I'm a 61yr lady who knows not alot about cars but can't afford to put it in a garage nor can get it there.
It turns over but won't start. Yes it seems the battery has seen better days but my hubby and I put my perfectly good battery from my Peugeot partner on it and it still won't start. There was loads of air in the clear fuel tubes next to the rubber bubble thing that you squeeze ( where on earth could that much air come from please ? ) We think we have now cleared that, but not sure, no more visible air in those clear pipes but it could I suppose have travelled further along the system. Still not starting but turns over. Next a friend suggested we crack off the injectors so that there is fuel there. We are doing this tomorrow night, we've got the casing off and will continue tomorrow. Don't want to be skipping too far ahead but are there any other obvious things that anyone could suggest please. We really do need to get this sorted as cheaply as possible so your ideas and help would be really appreciated. PS/ we bought a 2nd hand fuel pump but this has not helped at all, now don;t want to really keep guessing and buying parts to no avail. Thank you, Julie