Ask an expert

In Feb this year had a problem 'loosing power' it was checked to find oil leaks into spark plugs thru rocker cover.
New rocker cover gasket kit including fitting new spark plugs & fitting
second hand ignition coil pack. paid over $600 at that time.
Last week the same thing happened. (10 months later.)
again their check founds 3rd lead faulty replaced new set of spark plug leads. $245 paid.
Two days later after filling up, checking oil and water, 10mins into journey
again same problem. Returning immediately to mechanic he found the
cap on the water tank was missing.... could I foolishly not have put it back
on securely! now overheating.... replacing new cap advised to go home and bring back Monday. Monday given a car to use, Tuesday after not hearing from them, 2pm found them flushing water after which they could
tell if the head gasket had blown..... still today no ph call as yet.
Still no answer to why it was loosing power again.
Being elder lady not up with 'mechanic speak' would appreciate your opinion.

In trying to read between the lines here, it doesn't seem that all these things are related. Correct me if I'm wrong, but the way I read it is;
1) First issue with car running poorly/losing power was oil leak from rocker cover gasket, requiring new gasket, plugs and coil pack.
2) Second issue with same symptom was a faulty ignition lead.
3) Third issue seems likely that the cap was left off during routing oil and water check, which caused car to overheat.

Unfortunately, overheating can cause the head gasket to blow, but not always. Hopefully no damage was done, in which case you might only need a coolant change and new cap, however, if the head gasket is blown it will be a very expensive job.