I run Cory's Lawn Service here in Reno and have been doing lawn care since 2006--while I'm US-based, the turf principles translate. The biggest difference I see is you're working with cool-season grasses like we do (fescue, bluegrass, ryegrass), so my experience applies directly. Never lay turf over grass. We see this attempted maybe twice a year, and it always fails because the old root zone creates a barrier--new roots can't penetrate soil, moisture gets trapped between layers, and fungal diseases like dollar spot or red thread explode in those conditions. With 800+ five-star reviews, our most common fix for patchy lawns is aggressive overseeding combined with aeration, not replacement turf. We punch holes through compacted soil in early fall, spread seed at double the normal rate in bare zones, and get 70-80% coverage within three weeks at a fraction of turf cost. Lawns go patchy from three things we track: excessive thatch buildup (over half an inch blocks water and air), shallow watering that creates weak roots, and mowing too short which stresses grass and invites weeds. Our dethatching jobs in Reno consistently pull 2-4 inches of matted organic matter that homeowners didn't know existed--once removed and properly watered deep (1 inch per week, early morning), those dead zones fill back in naturally. If you're reseeding, skip the turf expense and invest in a soil test first; low nitrogen is the number one cause of thin, disease-prone grass we encounter.
I run Lawn Care Plus here in Massachusetts, and I've been doing lawn installations across Greater Boston for over a decade. The UK climate is actually pretty similar to ours--cool, damp, lots of shade--so what works here should translate well. One thing I rarely see mentioned is matching grass TYPE when you're patching. We always do a quick check of whether the existing lawn is Kentucky Bluegrass, Fescue, or a mix before laying any sod patches. I had a Newton client who patched with the wrong variety and six months later those sections were visibly different colors and textures--looked worse than the original bald spots. For patches, I'd only use sod if you're 100% certain you can match the existing grass species, otherwise seed blends in invisibly. The biggest mistake I see is skipping soil prep entirely. Before any sod goes down, we rake loose the top layer of existing soil--even if you've removed old turf--then spread a thin layer of compost or quality topsoil. This gives new roots something to grab onto immediately. On a Brookline project last spring, we laid 800 square feet of sod over properly prepped soil and it rooted in 10 days instead of the usual 3 weeks because those roots had ideal contact from day one. For timing, don't lay sod when it's too hot or bone dry. Early spring or fall works best in our climate--ground is still moist, temps are mild, and the grass establishes faster with less watering. We did a job in August once during a dry spell and had to water twice daily for three weeks straight just to keep it alive.
When people ask whether laying new turf over existing grass is a clever fix, I explain that it usually isn't worth the time or effort. Laying turf over grass creates a barrier that stops roots from bonding with the soil, so the new lawn often dries out or dies unevenly. You can technically turf an entire lawn, but only after removing the old grass, improving the soil, and leveling the ground. I've seen homeowners try to shortcut this step and end up redoing the lawn within a year because the turf never properly established. Using sections of turf to patch bald spots can work, but only if the underlying soil is healthy and the surrounding grass is strong. In many cases, reseeding with quality grass seed is more effective and blends better over time, especially for small or irregular patches. Lawns usually go patchy due to compacted soil, poor drainage, pet urine, shade issues, or inconsistent mowing and feeding. One job that stuck with me involved a lawn that kept failing until we discovered heavy foot traffic had compacted the soil; aeration and reseeding solved what turf patches never could. To properly replace a lawn, old turf needs to be removed completely using a turf cutter or spade, taking off the grass and root layer down to clean soil. From there, the ground should be loosened, enriched, leveled, and lightly firmed before laying new turf. The biggest lesson from my experience is that preparation matters more than the turf itself. When the soil is right, both turf and seed perform well; when it's ignored, even the best turf struggles.