Read your local university extension's recommendations about when to apply pre-emergent herbicide. I'm doing a split application of dimension this year.

Get a soil test. Do what the results say. The turf can't compete against weeds if it's not healthy.

Fertilize according to the grass type you have & when it starts waking up in your area. (Again, your local Ag extension is your friend here.)

A week or two later, spray herbicide. Spot or blanket, depending on how bad the weeds are.

Mow fescue at ~4inches. That's probably the highest or next to highest setting on your mower.

More info on someone smarter than me:

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There Is No Magic

Despite what many suppliers of weed killers will tell you, there is no single weed killer that will kill all types of weeds without harming your lawn. There are weed killers for diiferent broadleaf weeds, and others for grassy weeds.

And while they may be easy, "weed and feed" products don't work as well as herbicide-only products - I'll discuss why later in this article.

When to Tackle Weeds

The best time to tackle a plethora of weeds is when most of them are actively growing. In the North, this is generally late April until Memorial Day, and then early Fall. Summer is not the best time to tackle them - grasses and most weeds are in a slower "survival mode" waiting for the best growing conditions of Fall.

The Triangle Approach

Picture a triangle with the fat part on the bottom. We don't want to bother figuring out what weeds you have if your lawn is infested with 20 different weeds. We want to eliminate most of them with an inexpensive, readily-available, easy-to-apply weed killer. First, we apply a fertilizer to get them growing, This causes a growth spurt, and the metabolism goes up. Somewhere between Days 10-14 this tendency peaks (it is longer for organic fertilizers). "Weed-and-Feed" products can't take advantage of this growth spurt. This is the moment to strike - hit the weeds with a general-purpose broadleaf liquid weed killer containing 2,4D - possibly in combination with Dicamba and/or MCPP. This is a broad-spectrum kill that will get lots of easy-to-kill weeds. Generally it is a fairly fast kill of 7-14 days. Brand names do not matter - the ingredients do. If you had a lot of weeds, you may repeat the treatment a second time.

The middle part of the triangle is for the tougher weeds that don't knuckle under to broad-spectrum killers. If you started with 20 weed types, you'll probably be down to 3-4 types now. These will generally be things like Clover, Chickweed, Oxalis, Wild Violets and Ground Ivy. It's a good idea to get these weeds identified by someone.... The weed-killers needed will not be household names, but will be widely-available. The kills will be slower - often 2-3 treatments 14 days apart, with nothing visibly damaged for a long time. Stay the course - the weed killers work. The weeds may just get smaller and weaker without yellowing.

The top of the triangle is the use of specialty herbicides. Often by this point you're down to just grassy weeds, but these are the hardest to beat. The weed killers will be names you often have never heard of, and they can be more expensive to buy, difficult to find, and exteremely touchy about being applied correctly. This phase may be one that some novice lawn owners skip - they choose just to live with grassy weeds as part of the permanent situation.
Source linked below (hope that's not a TOS violation...)

Here is a link that might be useful: Andy on ATY

Great stuff! Thank you!

One other thing - I bought the house a year ago and the back yard was brand new at the time so its been a work in progress overseeding/aerating ect which I will be doing again this year.

Do I want to avoid certain chemicals with new seed growing?

I was reading and thinking "gee, that material seems familiar" ;-)