To echo others. Filter isn't in the ideal place, but better there than not at all. Water tank looks like the old tank feeding the heating so it is no longer in use. How does the big tank look? That's the one that he should have cleaned. It doesn't look like the neatest install ever but you might be gunning for this guy when he is in the right. I would suggest maybe paying someone an hour's labour to come out and check over the system before you take anymore action. Depending on where you are £50-£100 might be a small price to pay to give you peace of mind and potentially causing you both a headache when it could be avoided. To be fair, I'd have the hump if someone employed another gas engineer days after I'd been there, that or if be questioning where I went wrong!